Make OffscreenCanvas an EventTarget

third_party/WebKit/Source/modules/offscreencanvas/OffscreenCanvasModules.cpp

 // 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.
 
 #include "modules/offscreencanvas/OffscreenCanvasModules.h"
 
 #include "core/html/canvas/CanvasContextCreationAttributes.h"
 #include "core/offscreencanvas/OffscreenCanvas.h"
 #include "modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.h"
 
 namespace blink {
 
 void OffscreenCanvasModules::getContext(
     ScriptState* scriptState,
     OffscreenCanvas& offscreenCanvas,
     const String& id,
     const CanvasContextCreationAttributes& attributes,
     ExceptionState& exceptionState,
     OffscreenRenderingContext& result) {
   if (offscreenCanvas.isNeutered()) {
     exceptionState.throwDOMException(InvalidStateError,
                                      "OffscreenCanvas object is detached");
     return;
   }
 
+  offscreenCanvas.setExecutionContext(scriptState->getExecutionContext());

[Justin Novosad, 2016/11/14 19:45:08]

Add a comment: OffscreenCanvas cannot be transferred after getContext, so this
execution context will always be the right one from here on.

   CanvasRenderingContext* context =
       offscreenCanvas.getCanvasRenderingContext(scriptState, id, attributes);
   if (context)
     context->setOffscreenCanvasGetContextResult(result);
 }
 
 }  // namespace blink