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.

index.html

@@ -364,6 +364,7 @@ <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;
@@ -388,6 +389,11 @@ <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
@@ -549,6 +555,32 @@ <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 class='note'>
+            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=
@@ -1062,7 +1094,8 @@ <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|, and a [=/response body info=] |bodyInfo|, perform the
+          |cacheMode|, a [=/response body info=] |bodyInfo|, and an optional
+          [=string=] |deliveryType| (by default, the empty string), perform the
           following steps:
         </p>
         <ol>
@@ -1071,8 +1104,8 @@ <h2>
           </li>
           <li>
             <a>Setup the resource timing entry</a> for |entry|, given
-            |initiatorType|, |requestedURL|, |timingInfo|, |cacheMode|, and
-            |bodyInfo|.
+            |initiatorType|, |requestedURL|, |timingInfo|, |cacheMode|,
+            |bodyInfo|, and |deliveryType|.
           </li>
           <li>
             <a data-cite=
@@ -1089,8 +1122,9 @@ <h2>
           To <dfn data-export="">setup the resource timing entry</dfn> for
           <a>PerformanceResourceTiming</a> |entry| given DOMString
           |initiatorType|, DOMString |requestedURL|, [=/fetch timing info=]
-          |timingInfo|, a DOMString |cacheMode|, and a [=response body info=]
-          |bodyInfo|, perform the following steps:
+          |timingInfo|, a DOMString |cacheMode|, a [=response body info=]
+          |bodyInfo|, and an optional DOMString |deliveryType| (by default, the
+          empty string), perform the following steps:
         </p>
         <ol>
           <li>Assert that |cacheMode| is the empty string or
@@ -1111,6 +1145,12 @@ <h2>
           <li>Set |entry|'s <a data-for="PerformanceResourceTiming">cache
           mode</a> to |cacheMode|.
           </li>
+          <li>If |deliveryTode| is the empty string and |cacheMode| is not,
+          then set |deliveryTode| 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}}