Prevent <canvas> destructor from causing nested garbage collection

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 549 matching lines @@
     m_externallyAllocatedMemory = externallyAllocatedMemory;
 }
 
 GraphicsContext* HTMLCanvasElement::drawingContext() const
 {
     return buffer() ? m_imageBuffer->context() : 0;
 }
 
 GraphicsContext* HTMLCanvasElement::existingDrawingContext() const
 {
-    if (m_didFailToCreateImageBuffer) {
-        ASSERT(!hasImageBuffer());
+    if (!hasImageBuffer())
         return 0;
-    }
 
     return drawingContext();
 }
 
 ImageBuffer* HTMLCanvasElement::buffer() const
 {
     if (!hasImageBuffer() && !m_didFailToCreateImageBuffer)
         const_cast<HTMLCanvasElement*>(this)->createImageBuffer();
     return m_imageBuffer.get();
 }
@@ skipping 111 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }