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

cc/resources/gpu_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_GPU_RASTERIZER_H_
 #define CC_RESOURCES_GPU_RASTERIZER_H_
 
 #include <vector>
 
 #include "cc/base/cc_export.h"
 #include "cc/resources/rasterizer.h"
 #include "cc/resources/resource_pool.h"
 #include "cc/resources/tile.h"
 #include "third_party/skia/include/core/SkMultiPictureDraw.h"
 
 namespace cc {
 
 class ContextProvider;
 class ResourceProvider;
 
 class CC_EXPORT GpuRasterizer : public Rasterizer {
  public:
   ~GpuRasterizer() override;
 
-  static scoped_ptr<GpuRasterizer> Create(ContextProvider* context_provider,
-                                          ResourceProvider* resource_provider,
-                                          bool use_distance_field_text,
-                                          bool tile_prepare_enabled,
-                                          int msaa_sample_count);
+  static scoped_ptr<GpuRasterizer> Create(
+      ContextProvider* context_provider,
+      ResourceProvider* resource_provider,
+      bool use_distance_field_text,
+      bool threaded_gpu_rasterization_enabled,
+      int msaa_sample_count);
   PrepareTilesMode GetPrepareTilesMode() override;
   void RasterizeTiles(
       const TileVector& tiles,
       ResourcePool* resource_pool,
       ResourceFormat resource_format,
       const UpdateTileDrawInfoCallback& update_tile_draw_info) override;
 
  private:
   GpuRasterizer(ContextProvider* context_provider,
                 ResourceProvider* resource_provider,
                 bool use_distance_filed_text,
-                bool tile_prepare_enabled,
+                bool threaded_gpu_rasterization_enabled,
                 int msaa_sample_count);
 
   using ScopedResourceWriteLocks =
       ScopedPtrVector<ResourceProvider::ScopedWriteLockGr>;
 
   void PerformSolidColorAnalysis(const Tile* tile,
                                  RasterSource::SolidColorAnalysis* analysis);
   void AddToMultiPictureDraw(const Tile* tile,
                              const ScopedResource* resource,
                              ScopedResourceWriteLocks* locks);
 
   ContextProvider* context_provider_;
   ResourceProvider* resource_provider_;
   SkMultiPictureDraw multi_picture_draw_;
 
   bool use_distance_field_text_;
-  bool tile_prepare_enabled_;
+  bool threaded_gpu_rasterization_enabled_;
   int msaa_sample_count_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuRasterizer);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_GPU_RASTERIZER_H_