Use DrawingRecorder::canUseCachedDrawing

Source/core/paint/RenderDrawingRecorder.h

Index: Source/core/paint/RenderDrawingRecorder.h
diff --git a/Source/core/paint/RenderDrawingRecorder.h b/Source/core/paint/RenderDrawingRecorder.h
index 5acd0fd08bfaa61c9846311ea8fe08e4c25a186d..6254106b5d6a3ab756791218068c069d4ddbf6b6 100644
--- a/Source/core/paint/RenderDrawingRecorder.h
+++ b/Source/core/paint/RenderDrawingRecorder.h
@@ -15,16 +15,15 @@ namespace blink {
 class GraphicsContext;
 class RenderObject;
 
-class RenderDrawingRecorder {
+class RenderDrawingRecorder : public DrawingRecorder {

[Xianzhu, 2014/12/11 19:50:59]

This is to inherit the canUseCachedDrawing method. Otherwise we would need to
delegate it.

[chrishtr, 2014/12/12 00:18:08]

Let's delegate instead. That pattern is already in place, for the reason that
it's cleaner to reason about composition rather than inheritance,
especially regarding member variables.

[Xianzhu, 2014/12/12 00:56:49]

Done.

 public:
-    explicit RenderDrawingRecorder(GraphicsContext*, const RenderObject*, PaintPhase, const FloatRect&);
+    explicit RenderDrawingRecorder(GraphicsContext*, const RenderObject&, PaintPhase, const FloatRect&);
 
     ~RenderDrawingRecorder();
 
 private:
-    DrawingRecorder drawingRecorder;
 #ifndef NDEBUG
-    const RenderObject* m_renderer;
+    const RenderObject& m_renderer;
 #endif
 };