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

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 92a6cd515bded43a2ff0e3dfdf953f0ddba134d5..7b67a54e355ebedd05e413a781beb8124850b8f1 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -598,6 +598,8 @@ void RenderThreadImpl::Init() {
   } else {
     gpu_rasterization_msaa_sample_count_ = 0;
   }
+  is_threaded_gpu_rasterization_enabled_ =
+      command_line.HasSwitch(switches::kEnableThreadedGpuRasterization);
 
   if (command_line.HasSwitch(switches::kDisableDistanceFieldText)) {
     is_distance_field_text_enabled_ = false;
@@ -1386,9 +1388,14 @@ bool RenderThreadImpl::UseSingleThreadScheduler() {
   return !cmd->HasSwitch(switches::kDisableSingleThreadProxyScheduler);
 }
 
+bool RenderThreadImpl::IsThreadedGpuRasterizationEnabled() {
+  return is_threaded_gpu_rasterization_enabled_;
+}
+
 uint32 RenderThreadImpl::GetImageTextureTarget() {
   return use_image_texture_target_;
 }
+
 scoped_refptr<base::SingleThreadTaskRunner>
 RenderThreadImpl::GetCompositorMainThreadTaskRunner() {
   return main_thread_compositor_task_runner_;