Make OffscreenCanvasRenderingContext2D renderable on a worker

third_party/WebKit/Source/modules/offscreencanvas/OffscreenCanvas.idl

 // Copyright 2015 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: If there're more context types implemented, they should be added here
 // to form a union type of OffscreenCanvasRenderingContext
 typedef OffscreenCanvasRenderingContext2D OffscreenRenderingContext;
 
 [
     Constructor([EnforceRange] unsigned long width, [EnforceRange] unsigned long height),
     GarbageCollected,
     Exposed=(Window,Worker),
     RuntimeEnabled=ExperimentalCanvasFeatures,
 ] interface OffscreenCanvas {
     [EnforceRange] attribute unsigned long width;
     [EnforceRange] attribute unsigned long height;
 
-    OffscreenRenderingContext? getContext(DOMString contextId, optional CanvasContextCreationAttributes attributes); 
+    OffscreenRenderingContext? getContext(DOMString contextId, optional CanvasContextCreationAttributes attributes);
+    [RaisesException] ImageBitmap transferToImageBitmap();
 };