Handle exception when user call canvas.transferControlToOffscreen() twice

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 1332 matching lines @@
   return HitTestCanvasResult::create(String(), nullptr);
 }
 
 String HTMLCanvasElement::getIdFromControl(const Element* element) {
   if (m_context)
     return m_context->getIdFromControl(element);
   return String();
 }
 
 bool HTMLCanvasElement::createSurfaceLayer() {
-  DCHECK(!m_surfaceLayerBridge);

[Justin Novosad, 2016/11/02 20:00:11]

I think this DCHECK should stay.  It was probably firing before you made the fix
in HTMLCanvasElementModule.cpp, but now it should be Okay.

[xlai (Olivia), 2016/11/02 20:07:50]

Done.

   mojom::blink::OffscreenCanvasSurfacePtr service;
   Platform::current()->interfaceProvider()->getInterface(
       mojo::GetProxy(&service));
   m_surfaceLayerBridge =
       wrapUnique(new CanvasSurfaceLayerBridge(std::move(service)));
   return m_surfaceLayerBridge->createSurfaceLayer(this->width(),
                                                   this->height());
 }
 
 }  // namespace blink