Use WebCompositor only when --enable-threaded-compositing.

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 ee9615ab1ae83e115f652fe734fc6c667e55103a..9871a1754ad7c14c564a5aa4accb9ece93524543 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -431,8 +431,11 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
   webkit_platform_support_.reset(new RendererWebKitPlatformSupportImpl);
   WebKit::initialize(webkit_platform_support_.get());
 
-  compositor_thread_.reset(new CompositorThread(this));
-  AddFilter(compositor_thread_->GetMessageFilter());
+  if (CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kEnableThreadedCompositing)) {
+    compositor_thread_.reset(new CompositorThread(this));
+    AddFilter(compositor_thread_->GetMessageFilter());
+  }
 
   WebScriptController::enableV8SingleThreadMode();