Have Document::getCSSCanvasElement() return a reference

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 59 matching lines @@
 IntSize CSSCanvasValue::fixedSize(const RenderObject* renderer)
 {
     if (HTMLCanvasElement* elt = element(&renderer->document()))
         return IntSize(elt->width(), elt->height());
     return IntSize();
 }
 
 HTMLCanvasElement* CSSCanvasValue::element(Document* document)
 {
      if (!m_element) {
-        m_element = document->getCSSCanvasElement(m_name);
-        if (!m_element)
-            return 0;
+        m_element = &document->getCSSCanvasElement(m_name);
         m_element->addObserver(&m_canvasObserver);
     }
     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())
         return nullptr;
     return elt->copiedImage();
 }
 
 bool CSSCanvasValue::equals(const CSSCanvasValue& other) const
 {
     return m_name == other.m_name;
 }
 
 } // namespace WebCore