cc: Setup API to release OutputSurface from LTHClient.

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index d403ae0eb6da1ebce0df41dc64215daa9a252376..7bdca5dc2fe3b84d2af5619d19ec85bdc9501aba 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -62,6 +62,8 @@ class CC_EXPORT Proxy {
   // with the result of this function.
   virtual void SetOutputSurface(scoped_ptr<OutputSurface> output_surface) = 0;
 
+  virtual scoped_ptr<OutputSurface> ReleaseOutputSurface() = 0;
+
   // Indicates that the compositing surface associated with our context is
   // ready to use.
   virtual void SetLayerTreeHostClientReady() = 0;