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

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index acdaa779949c00bbfe2008b5b3b7d55ca05575be..2277a5c9f63fd3d4e197fe267d4a697d7b4a73ca 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -1081,6 +1081,9 @@ static void AppendCompositorCommandLineFlags(base::CommandLine* command_line) {
   if (IsGpuRasterizationEnabled())
     command_line->AppendSwitch(switches::kEnableGpuRasterization);
 
+  if (IsThreadedGpuRasterizationEnabled())
+    command_line->AppendSwitch(switches::kEnableThreadedGpuRasterization);
+
   int msaa_sample_count = GpuRasterizationMSAASampleCount();
   if (msaa_sample_count > 0) {
     command_line->AppendSwitchASCII(