Use DrawingRecorder::canUseCachedDrawing

Source/core/paint/RenderDrawingRecorder.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef RenderDrawingRecorder_h
 #define RenderDrawingRecorder_h
 
 #include "core/rendering/PaintPhase.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/graphics/paint/DrawingRecorder.h"
 
 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
 };
 
 } // namespace blink
 
 #endif // RenderDrawingRecorder_h