Add rendering context and rendering 2D context to OffscreenCanvas

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.idl

 // Copyright 2016 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.
 
+// Note: The official spec is WIP at wiki.whatwg.org/wiki/OffscreenCanvas.
+
 [
+    GarbageCollected,
+    Exposed=(Window,Worker),
     RuntimeEnabled=ExperimentalCanvasFeatures,
-    SetWrapperReferenceFrom=canvas,
-    WillBeGarbageCollected,
-] interface ImageBitmapRenderingContext {
+] interface OffscreenCanvasRenderingContext2D {
     // back-reference to the canvas
-    readonly attribute HTMLCanvasElement canvas;
+    readonly attribute OffscreenCanvas offscreenCanvas;
+};
 
-    void transferImageBitmap(ImageBitmap bitmap);
-};
+OffscreenCanvasRenderingContext2D implements CanvasPathMethods;