Fix bug where setting canvas size to 0 was treated as if context was lost

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 674 matching lines @@
             return surface.release();
         surfaceFactory = adoptPtr(new UnacceleratedSurfaceFactory()); // recreate because previous one was released
     }
 
     return surfaceFactory->createSurface(deviceSize, opacityMode);
 }
 
 void HTMLCanvasElement::createImageBuffer()
 {
     createImageBufferInternal(nullptr);
-    if (m_didFailToCreateImageBuffer && m_context->is2d())
+    if (m_didFailToCreateImageBuffer && m_context->is2d() && !size().isEmpty())
         m_context->loseContext(CanvasRenderingContext::SyntheticLostContext);
 }
 
 void HTMLCanvasElement::createImageBufferInternal(PassOwnPtr<ImageBufferSurface> externalSurface)
 {
     ASSERT(!m_imageBuffer);
 
     m_didFailToCreateImageBuffer = true;
     m_imageBufferIsClear = true;
 
@@ skipping 237 matching lines @@
 {
     return FloatSize(width(), height());
 }
 
 bool HTMLCanvasElement::isOpaque() const
 {
     return m_context && !m_context->hasAlpha();
 }
 
 } // blink