[Fetch] Add a deprecation warning for Request.context

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",
@@ skipping 12 matching lines @@
     ConstructorCallWith=ScriptState,
     Exposed=(Window,Worker),
     RaisesException=Constructor,
     ActiveDOMObject,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Request {
     readonly attribute ByteString method;
     readonly attribute USVString url;
     readonly attribute Headers headers;
-    readonly attribute RequestContext context;
+    [DeprecateAs=FetchAPIRequestContext] readonly attribute RequestContext context;
     readonly attribute DOMString referrer;
     readonly attribute RequestMode mode;
     readonly attribute RequestCredentials credentials;
     // TODO(horo): Support redirect attribute when the chromium side patch land.
     // readonly attribute RequestRedirect redirect;
 
     [RaisesException] Request clone();
 };
 
 Request implements Body;