cc: Add testing stubs for proxy test

cc/trees/thread_proxy.h

Index: cc/trees/thread_proxy.h
diff --git a/cc/trees/thread_proxy.h b/cc/trees/thread_proxy.h
index e2a8f21c57421b9eee46f4c987b49acb9ef1b92a..a3b3bcbbbb0630ff75892717bc946bb1acbb7f6d 100644
--- a/cc/trees/thread_proxy.h
+++ b/cc/trees/thread_proxy.h
@@ -30,8 +30,9 @@ class LayerTreeHost;
 class ResourceUpdateQueue;
 class Scheduler;
 class ScopedThreadProxy;
+class ThreadProxyTestAPI;
 
-class ThreadProxy : public Proxy,
+class CC_EXPORT ThreadProxy : public Proxy,
                     LayerTreeHostImplClient,
                     SchedulerClient,
                     ResourceUpdateControllerClient {
@@ -122,10 +123,13 @@ class ThreadProxy : public Proxy,
   // ResourceUpdateControllerClient implementation
   virtual void ReadyToFinalizeTextureUpdates() OVERRIDE;
 
- private:
+ protected:
   ThreadProxy(LayerTreeHost* layer_tree_host,
               scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner);
 
+ private:
+  friend class ThreadProxyTestAPI;
+
   struct BeginMainFrameAndCommitState {
     BeginMainFrameAndCommitState();
     ~BeginMainFrameAndCommitState();