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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 04d0c94d6753ceb03f95d985b60a13def01c2094..c97a5fdd01650847112a9dfdb45398e8bd9e635e 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1960,7 +1960,8 @@ 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();