Add content-type to resource-timing (#341)

* Add content-type to resource-timing

* Move content-type into response body info

* Mention possibility of truncating by user agents

* Add link to user agent

* Remove stripping

* Serialize instead of returning essence

* Return minimzed content type received from fetch

Author: abinpaul1

index.html

@@ -375,6 +375,7 @@ <h3>
               readonly attribute unsigned long long  decodedBodySize;
               readonly attribute unsigned short responseStatus;
               readonly attribute RenderBlockingStatusType renderBlockingStatus;
+              readonly attribute DOMString contentType;
               [Default] object toJSON();
           };
         </pre>
@@ -757,6 +758,11 @@ <h3>
           "FETCH#concept-filtered-response-opaque">opaque filtered
           response</a>.
         </p>
+        <p data-dfn-for="PerformanceResourceTiming">
+          The <dfn>contentType</dfn> getter steps are to return
+          <a>this</a>'s <a data-for="PerformanceResourceTiming">resource
+          info</a>'s [=response body info/content type=].
+        </p>
         <p data-dfn-for="PerformanceResourceTiming">
           The <dfn>renderBlockingStatus</dfn> getter steps are to return
           <a data-link-for="RenderBlockingStatusType">blocking</a> if