Add delivery type to PerformanceResourceTiming. (#343)

* Add delivery type to PerformanceResourceTiming.

This defines a concept of "delivery type" which currently describes the
difference between resources delivered from the HTTP cache and those not
(currently only detectable via transferSize). It is expected to be
extended to describe resources delivered from other stores, such as the
consuming resources from the HTML preload buffer and navigational
prefetches.

* add deliveryType to the list of attributes a UA may obscure

index.html

@@ -355,6 +355,7 @@ <h3>
           [Exposed=(Window,Worker)]
           interface PerformanceResourceTiming : PerformanceEntry {
               readonly attribute DOMString initiatorType;
+              readonly attribute DOMString deliveryType;
               readonly attribute ByteString nextHopProtocol;
               readonly attribute DOMHighResTimeStamp workerStart;
               readonly attribute DOMHighResTimeStamp redirectStart;
@@ -381,6 +382,11 @@ <h3>
           <a data-dfn-for="PerformanceResourceTiming"><dfn>initiator
           type</dfn></a>.
         </p>
+        <p>
+          A <a>PerformanceResourceTiming</a> has an associated DOMString
+          <a data-dfn-for="PerformanceResourceTiming"><dfn>delivery
+          type</dfn></a>.
+        </p>
         <p>
           A <a>PerformanceResourceTiming</a> has an associated DOMString
           <a data-dfn-for="PerformanceResourceTiming"><dfn>requested
@@ -552,6 +558,32 @@ <h3>
           The setting of `initiatorType` is done at the different places where
           a resource timing entry is reported, such as the [=fetch=] standard.
         </p>
+        <p data-dfn-for="PerformanceResourceTiming">
+          <dfn>deliveryType</dfn> getter steps are to return the <a data-for=
+          "PerformanceResourceTiming">delivery type</a> for <a>this</a>.
+        </p>
+        <div class='note'>
+          <p>
+            `deliveryType` returns one of the following values:
+          </p>
+          <ul>
+            <li>
+              <code>"cache"</code>, if the <a data-for=
+              "PerformanceResourceTiming">cache mode</a> is not the empty
+              string.
+            </li>
+            <li>
+              the empty string <code>""</code>, if none of the above conditions
+              match.
+            </li>
+          </ul>
+
+          <p>
+            This is expected to be expanded by future updates to this
+            specification, e.g. to describe consuming preloaded resources and
+            prefetched navigation requests.
+          </p>
+        </div>
         <p data-dfn-for="PerformanceResourceTiming">
           The <dfn>workerStart</dfn> getter steps are to <a>convert fetch
           timestamp</a> for <a>this</a>'s <a data-for=
@@ -1030,7 +1062,8 @@ <h4>
           <p>
             The user agent MAY still enforce cross-origin restrictions and set
             transferSize, encodedBodySize, and decodedBodySize attributes to
-            zero, even with Timing-Allow-Origin HTTP response header fields.
+            zero, even with Timing-Allow-Origin HTTP response header fields. If
+            it does, it MAY also set deliveryType to "".
           </p>
           <p>
             The <a>Timing-Allow-Origin</a> headers are processed in
@@ -1130,8 +1163,9 @@ <h2>
           To <dfn data-export="">mark resource timing</dfn> given a [=/fetch
           timing info=] |timingInfo|, a DOMString |requestedURL|, a DOMString
           |initiatorType| a <a>global object</a> |global|, a string
-          |cacheMode|, a [=/response body info=] |bodyInfo|, and a
+          |cacheMode|, a [=/response body info=] |bodyInfo|, a
           <a data-cite="FETCH#concept-status">status</a> |responseStatus|,
+          and an optional [=string=] |deliveryType| (by default, the empty string),
           perform the following steps:
         </p>
         <ol>
@@ -1141,7 +1175,7 @@ <h2>
           <li>
             <a>Setup the resource timing entry</a> for |entry|, given
             |initiatorType|, |requestedURL|, |timingInfo|, |cacheMode|,
-            |bodyInfo| and |responseStatus|.
+            |bodyInfo|, |responseStatus|, and |deliveryType|.
           </li>
           <li>
             <a data-cite=
@@ -1159,8 +1193,9 @@ <h2>
           <a>PerformanceResourceTiming</a> |entry| given DOMString
           |initiatorType|, DOMString |requestedURL|, [=/fetch timing info=]
           |timingInfo|, a DOMString |cacheMode|, a [=response body info=]
-          |bodyInfo|, and a <a data-cite="FETCH#concept-status">status</a>
-          |responseStatus|, perform the following steps:
+          |bodyInfo|, a <a data-cite="FETCH#concept-status">status</a>
+          |responseStatus|, and an optional DOMString |deliveryType| (by
+          default, the empty string), perform the following steps:
         </p>
         <ol>
           <li>Assert that |cacheMode| is the empty string,
@@ -1184,6 +1219,12 @@ <h2>
           <li>Set |entry|'s <a data-for="PerformanceResourceTiming">response
           status</a> to |responseStatus|.
           </li>
+          <li>If |deliveryType| is the empty string and |cacheMode| is not,
+          then set |deliveryType| to "<code>cache</code>".
+          </li>
+          <li>Set |entry|'s <a data-for="PerformanceResourceTiming">delivery
+          type</a> to |deliveryType|.
+          </li>
         </ol>
         <p>
           To <dfn>convert fetch timestamp</dfn> given {{DOMHighResTimeStamp}}