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

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index cc5e148c27fc39f4b0cc7fd0c62c7199be43c933..8142ee4dc35444ec265da562c490922567b5f3d8 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -97,14 +97,13 @@ 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();
 }