Add non-blocking frame rate limiting logic to display list 2D canvas

Source/platform/graphics/ImageBuffer.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * 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:
  *
  *     * Redistributions of source code must retain the above copyright
@@ skipping 79 matching lines @@
 }
 
 ImageBuffer::~ImageBuffer()
 {
 }
 
 GraphicsContext* ImageBuffer::context() const
 {
     if (!isSurfaceValid())
         return 0;
-    m_surface->willUse();
     ASSERT(m_context.get());
     return m_context.get();
 }
 
 const SkBitmap& ImageBuffer::bitmap() const
 {
-    m_surface->willUse();
+    m_surface->didDraw(); // conservative
     return m_surface->bitmap();
 }
 
 bool ImageBuffer::isSurfaceValid() const
 {
     return m_surface->isValid();
 }
 
 bool ImageBuffer::restoreSurface() const
 {
@@ skipping 304 matching lines @@
     if (!encodeImage(imageData, mimeType, quality, &encodedImage))
         return "data:,";
 
     Vector<char> base64Data;
     base64Encode(encodedImage, base64Data);
 
     return "data:" + mimeType + ";base64," + base64Data;
 }
 
 } // namespace blink