Introduce separate client and init path for single-threaded cc

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index e5b706f07e643afc365d732d3f846c4b54ec6aff..7fd822ed42d90456598ec9c3c9d1117695b11d8f 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -17,10 +17,13 @@ namespace cc {
 
 class ContextProvider;
 class LayerTreeHost;
+class LayerTreeHostSingleThreadClient;
 
 class SingleThreadProxy : public Proxy, LayerTreeHostImplClient {
  public:
-  static scoped_ptr<Proxy> Create(LayerTreeHost* layer_tree_host);
+  static scoped_ptr<Proxy> Create(
+      LayerTreeHost* layer_tree_host,
+      LayerTreeHostSingleThreadClient* client);
   virtual ~SingleThreadProxy();
 
   // Proxy implementation
@@ -79,7 +82,8 @@ class SingleThreadProxy : public Proxy, LayerTreeHostImplClient {
   void CompositeImmediately(base::TimeTicks frame_begin_time);
 
  private:
-  explicit SingleThreadProxy(LayerTreeHost* layer_tree_host);
+  SingleThreadProxy(LayerTreeHost* layer_tree_host,
+                    LayerTreeHostSingleThreadClient* client);
 
   void OnOutputSurfaceInitializeAttempted(bool success);
   bool CommitAndComposite(base::TimeTicks frame_begin_time,
@@ -100,6 +104,7 @@ class SingleThreadProxy : public Proxy, LayerTreeHostImplClient {
 
   // Accessed on main thread only.
   LayerTreeHost* layer_tree_host_;
+  LayerTreeHostSingleThreadClient* client_;
   bool created_offscreen_context_provider_;
 
   // Holds the first output surface passed from Start. Should not be used for