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..c12a399aa69c9389745a4624ea20680c1140a74f 100644
--- a/content/test/fake_compositor_dependencies.cc
+++ b/content/test/fake_compositor_dependencies.cc
@@ -54,6 +54,10 @@ bool FakeCompositorDependencies::UseSingleThreadScheduler() {
   return use_single_thread_scheduler_;
 }
 
+bool FakeCompositorDependencies::IsThreadedGpuRasterizationEnabled() {
+  return false;
+}
+
 uint32 FakeCompositorDependencies::GetImageTextureTarget() {
   return GL_TEXTURE_2D;
 }