Introduce separate client and init path for single-threaded cc

ui/compositor/compositor.h

Index: ui/compositor/compositor.h
diff --git a/ui/compositor/compositor.h b/ui/compositor/compositor.h
index 8b0aba6be58289b422fe57736ae5bfd95e65b22e..348c022a0fe0d713255bf0d041753ed180e24200 100644
--- a/ui/compositor/compositor.h
+++ b/ui/compositor/compositor.h
@@ -13,6 +13,7 @@
 #include "base/observer_list.h"
 #include "base/time/time.h"
 #include "cc/trees/layer_tree_host_client.h"
+#include "cc/trees/layer_tree_host_single_thread_client.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/compositor/compositor_export.h"
 #include "ui/compositor/compositor_observer.h"
@@ -219,6 +220,7 @@ class COMPOSITOR_EXPORT DrawWaiterForTest : public ui::CompositorObserver {
 // view hierarchy.
 class COMPOSITOR_EXPORT Compositor
     : NON_EXPORTED_BASE(public cc::LayerTreeHostClient),
+      NON_EXPORTED_BASE(public cc::LayerTreeHostSingleThreadClient),
       public base::SupportsWeakPtr<Compositor> {
  public:
   explicit Compositor(gfx::AcceleratedWidget widget);
@@ -321,10 +323,14 @@ class COMPOSITOR_EXPORT Compositor
   virtual void DidCommit() OVERRIDE;
   virtual void DidCommitAndDrawFrame() OVERRIDE;
   virtual void DidCompleteSwapBuffers() OVERRIDE;
-  virtual void ScheduleComposite() OVERRIDE;
   virtual scoped_refptr<cc::ContextProvider>
       OffscreenContextProvider() OVERRIDE;
 
+  // cc::LayerTreeHostSingleThreadClient implementation.
+  virtual void ScheduleComposite() OVERRIDE;
+  virtual void DidPostSwapBuffers() OVERRIDE {}
+  virtual void DidAbortSwapBuffers() OVERRIDE {}
+
   int last_started_frame() { return last_started_frame_; }
   int last_ended_frame() { return last_ended_frame_; }