Implement Request.context for Fetch API

Source/modules/fetch/Request.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // http://fetch.spec.whatwg.org/#request-class
 
 typedef (Request or USVString) RequestInfo;
 
+enum RequestContext {
+    "", "audio", "beacon", "cspreport", "download", "embed", "eventsource", "favicon",
+    "fetch", "font", "form", "frame", "hyperlink", "iframe", "image", "imageset", "import",
+    "internal", "location", "manifest", "metarefresh", "object", "ping", "plugin",
+    "prefetch", "script", "serviceworker", "sharedworker",
+    "subresource", "style", "track", "video", "worker", "xmlhttprequest", "xslt"
+};
 enum RequestMode { "same-origin", "no-cors", "cors" };
 enum RequestCredentials { "omit", "same-origin", "include" };
 
 [
     Constructor(RequestInfo input, optional Dictionary requestInitDict),
     ConstructorCallWith=ExecutionContext,
     Exposed(Window GlobalFetch, DedicatedWorker GlobalFetch, SharedWorker GlobalFetch, ServiceWorker ServiceWorker),
     RaisesException=Constructor,
     ActiveDOMObject,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Request {
     readonly attribute ByteString method;
     readonly attribute USVString url;
     readonly attribute Headers headers;
-
+    readonly attribute RequestContext context;
     readonly attribute DOMString referrer;
     readonly attribute RequestMode mode;
     readonly attribute RequestCredentials credentials;
 
     [RaisesException] Request clone();
-
-    // FIXME: Implement the following:
-    // readonly attribute RequestContext context;
 };
 
 Request implements Body;