ui: Cache the output of View::OnPaint when the View isn't invalid.

cc/resources/drawing_display_item.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 CC_RESOURCES_DRAWING_DISPLAY_ITEM_H_
 #define CC_RESOURCES_DRAWING_DISPLAY_ITEM_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/base/cc_export.h"
 #include "cc/resources/display_item.h"
@@ skipping 15 matching lines @@
     return make_scoped_ptr(new DrawingDisplayItem(picture));
   }
 
   void Raster(SkCanvas* canvas, SkDrawPictureCallback* callback) const override;
 
   bool IsSuitableForGpuRasterization() const override;
   int ApproximateOpCount() const override;
   size_t PictureMemoryUsage() const override;
   void AsValueInto(base::trace_event::TracedValue* array) const override;
 
+  scoped_ptr<DrawingDisplayItem> Clone();
+
  protected:
   explicit DrawingDisplayItem(skia::RefPtr<SkPicture> picture);
 
  private:
   skia::RefPtr<SkPicture> picture_;
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_DRAWING_DISPLAY_ITEM_H_