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

content/test/fake_compositor_dependencies.cc

Index: content/test/fake_compositor_dependencies.cc
diff --git a/content/test/fake_compositor_dependencies.cc b/content/test/fake_compositor_dependencies.cc
index 1f090a0fc2aae512a250d8f264109bc90a5d3b0a..e33b7ef1ac9a7c468f2ad9f9ebeff5518083c3ed 100644
--- a/content/test/fake_compositor_dependencies.cc
+++ b/content/test/fake_compositor_dependencies.cc
@@ -26,6 +26,10 @@ bool FakeCompositorDependencies::IsGpuRasterizationEnabled() {
   return false;
 }
 
+bool FakeCompositorDependencies::IsThreadedGpuRasterizationEnabled() {
+  return false;
+}
+
 int FakeCompositorDependencies::GetGpuRasterizationMSAASampleCount() {
   return 0;
 }