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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1942 matching lines @@
                           tile_task_worker_pool_->AsTileTaskRunner(),
                           rasterizer_.get(), scheduled_raster_task_limit);
 
   UpdateTileManagerMemoryPolicy(ActualManagedMemoryPolicy());
 }
 
 scoped_ptr<Rasterizer> LayerTreeHostImpl::CreateRasterizer() {
   ContextProvider* context_provider = output_surface_->context_provider();
   if (use_gpu_rasterization_ && context_provider) {
     return GpuRasterizer::Create(context_provider, resource_provider_.get(),
-                                 settings_.use_distance_field_text, false,
+                                 settings_.use_distance_field_text,
+                                 settings_.threaded_gpu_rasterization_enabled,
                                  settings_.gpu_rasterization_msaa_sample_count);
   }
   return SoftwareRasterizer::Create();
 }
 
 void LayerTreeHostImpl::CreateResourceAndTileTaskWorkerPool(
     scoped_ptr<TileTaskWorkerPool>* tile_task_worker_pool,
     scoped_ptr<ResourcePool>* resource_pool,
     scoped_ptr<ResourcePool>* staging_resource_pool) {
   base::SingleThreadTaskRunner* task_runner =
@@ skipping 1484 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc