Make cc::DrawingDisplayItem hold a const SkPicture.

cc/playback/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_PLAYBACK_DRAWING_DISPLAY_ITEM_H_
 #define CC_PLAYBACK_DRAWING_DISPLAY_ITEM_H_
 
 #include <stddef.h>
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/base/cc_export.h"
 #include "cc/playback/display_item.h"
 #include "skia/ext/refptr.h"
 #include "ui/gfx/geometry/point_f.h"
 
 class SkCanvas;
 class SkPicture;
 
 namespace cc {
 
 class CC_EXPORT DrawingDisplayItem : public DisplayItem {
  public:
   DrawingDisplayItem();
-  explicit DrawingDisplayItem(skia::RefPtr<SkPicture> picture);
+  explicit DrawingDisplayItem(skia::RefPtr<const SkPicture> picture);
   explicit DrawingDisplayItem(const proto::DisplayItem& proto);
   explicit DrawingDisplayItem(const DrawingDisplayItem& item);
   ~DrawingDisplayItem() override;
 
   void ToProtobuf(proto::DisplayItem* proto) const override;
   void Raster(SkCanvas* canvas,
               const gfx::Rect& canvas_playback_rect,
               SkPicture::AbortCallback* callback) const override;
   void AsValueInto(const gfx::Rect& visual_rect,
                    base::trace_event::TracedValue* array) const override;
   size_t ExternalMemoryUsage() const override;
 
   int ApproximateOpCount() const;
   bool IsSuitableForGpuRasterization() const;
 
   void CloneTo(DrawingDisplayItem* item) const;
 
  private:
-  void SetNew(skia::RefPtr<SkPicture> picture);
+  void SetNew(skia::RefPtr<const SkPicture> picture);
 
-  skia::RefPtr<SkPicture> picture_;
+  skia::RefPtr<const SkPicture> picture_;
 };
 
 }  // namespace cc
 
 #endif  // CC_PLAYBACK_DRAWING_DISPLAY_ITEM_H_