cc: Only make num_threads - 1 SkPicture clones.

cc/resources/picture.h

 // Copyright 2012 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_PICTURE_H_
 #define CC_RESOURCES_PICTURE_H_
 
 #include <string>
 #include <utility>
 #include <vector>
@@ skipping 39 matching lines @@
       const SkTileGridPicture::TileGridInfo& tile_grid_info,
       bool gather_pixels_refs,
       int num_raster_threads);
   static scoped_refptr<Picture> CreateFromValue(const base::Value* value);
   static scoped_refptr<Picture> CreateFromSkpValue(const base::Value* value);
 
   gfx::Rect LayerRect() const { return layer_rect_; }
   gfx::Rect OpaqueRect() const { return opaque_rect_; }
 
   // Get thread-safe clone for rasterizing with on a specific thread.
-  scoped_refptr<Picture> GetCloneForDrawingOnThread(
-      unsigned thread_index) const;
+  Picture* GetCloneForDrawingOnThread(unsigned thread_index);
 
   // Has Record() been called yet?
   bool HasRecording() const { return picture_.get() != NULL; }
 
   // Apply this scale and raster the negated region into the canvas. See comment
   // in PicturePileImpl::RasterCommon for explanation on negated content region.
   int Raster(SkCanvas* canvas,
              SkDrawPictureCallback* callback,
              const Region& negated_content_region,
              float contents_scale);
@@ skipping 87 matching lines @@
   base::ThreadChecker raster_thread_checker_;
 
   friend class base::RefCountedThreadSafe<Picture>;
   friend class PixelRefIterator;
   DISALLOW_COPY_AND_ASSIGN(Picture);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_PICTURE_H_