Add rendering context and rendering 2D context to OffscreenCanvas

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 OffscreenCanvasRenderingContext;

[Justin Novosad, 2016/03/07 15:30:26]

We should call this enum just "OffscreenRenderingContext" to avoid confusing it
with the class of the same name. FWIW, the enum for canvas elements is just
called "RenderingContext".

+
 [
     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;
+
+    OffscreenCanvasRenderingContext? getContext(DOMString contextId, optional CanvasContextCreationAttributes attributes); 
 };