[Fetch] Add a deprecation warning for Request.context

Source/modules/fetch/Request.idl

Index: Source/modules/fetch/Request.idl
diff --git a/Source/modules/fetch/Request.idl b/Source/modules/fetch/Request.idl
index 7d226aa3b03616bd408a25cf3a4f5f4c12b5198a..83f67adc8194532afb39109ce0206b930d20f1e3 100644
--- a/Source/modules/fetch/Request.idl
+++ b/Source/modules/fetch/Request.idl
@@ -30,7 +30,7 @@ enum RequestCredentials { "omit", "same-origin", "include" };
     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;