Add a command-line flag to enable threaded GPU rasterization.

cc/resources/rasterizer.h

 // Copyright 2015 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_RASTERIZER_H_
 #define CC_RESOURCES_RASTERIZER_H_
 
 #include <vector>
 
 #include "cc/base/cc_export.h"
 #include "cc/resources/resource_pool.h"
 #include "cc/resources/tile.h"
 
 namespace cc {
 class TileManager;
 
 enum class PrepareTilesMode {
   RASTERIZE_PRIORITIZED_TILES,
-  PREPARE_PRIORITIZED_TILES,
   PREPARE_NONE
 };
 
 class CC_EXPORT Rasterizer {
  public:
   using TileVector = std::vector<Tile*>;
   using UpdateTileDrawInfoCallback =
       base::Callback<void(Tile*,
                           scoped_ptr<ScopedResource>,
                           const RasterSource::SolidColorAnalysis&)>;
 
   virtual ~Rasterizer() {}
   virtual PrepareTilesMode GetPrepareTilesMode() = 0;
   virtual void RasterizeTiles(
       const TileVector& tiles,
       ResourcePool* resource_pool,
       ResourceFormat resource_format,
       const UpdateTileDrawInfoCallback& update_tile_draw_info) = 0;
 
  protected:
   Rasterizer() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Rasterizer);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTERIZER_H_