Don't destroy RenderWidgetCompositor during DoDeferredClose

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index 112c259dc47c04bb6eb3f45f9c517390f885c741..3bd0e1da54cff631e9dcd6626cd5f21bcdbadf19 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -61,6 +61,10 @@ class CC_EXPORT Proxy {
   // ready to use.
   virtual void SetLayerTreeHostClientReady() = 0;
 
+  // Indicates that shutdown will happen soon and no more output surfaces
+  // should be created.
+  virtual void SetLayerTreeHostClientFinished() = 0;
+
   virtual void SetVisible(bool visible) = 0;
 
   virtual const RendererCapabilities& GetRendererCapabilities() const = 0;