Fix crash due to null frame() pointer on Document.

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 419 matching lines @@
 
     if (m_accelerationDisabled)
         return false;
 
     Settings* settings = document().settings();
     if (!settings || !settings->accelerated2dCanvasEnabled())
         return false;
 
     // Do not use acceleration for small canvases, unless GPU rasterization is available.
     // GPU raterization is a heuristic to avoid difficult content & whitelist targeted content.
-    if (!document().frame()->chromeClient().usesGpuRasterization() && size.width() * size.height() < settings->minimumAccelerated2dCanvasSize())
+    if (!(document().frame() && document().frame()->chromeClient().usesGpuRasterization()) && size.width() * size.height() < settings->minimumAccelerated2dCanvasSize())
         return false;
 
     if (!blink::Platform::current()->canAccelerate2dCanvas())
         return false;
 
     return true;
 }
 
 PassOwnPtr<ImageBufferSurface> HTMLCanvasElement::createImageBufferSurface(const IntSize& deviceSize, int* msaaSampleCount)
 {
@@ skipping 250 matching lines @@
 {
     return !originClean();
 }
 
 FloatSize HTMLCanvasElement::sourceSize() const
 {
     return FloatSize(width(), height());
 }
 
 }