canvas: make a temporary buffer when a context doesn't exist.

Source/core/css/CSSCanvasValue.cpp

 /*
  * Copyright (C) 2008 Apple Inc.  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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 71 matching lines @@
         m_element = &document->getCSSCanvasElement(m_name);
         m_element->addObserver(m_canvasObserver.get());
     }
     return m_element;
 }
 
 PassRefPtr<Image> CSSCanvasValue::image(RenderObject* renderer, const IntSize& /*size*/)
 {
     ASSERT(clients().contains(renderer));
     HTMLCanvasElement* elt = element(&renderer->document());
-    if (!elt || !elt->buffer())
+    if (!elt)
         return nullptr;
     return elt->copiedImage(FrontBuffer);
 }
 
 bool CSSCanvasValue::equals(const CSSCanvasValue& other) const
 {
     return m_name == other.m_name;
 }
 
 void CSSCanvasValue::traceAfterDispatch(Visitor* visitor)
 {
     visitor->trace(m_canvasObserver);
     visitor->trace(m_element);
     CSSImageGeneratorValue::traceAfterDispatch(visitor);
 }
 
 } // namespace blink