Provide a vsync notification to the renderer

cc/output_surface.h

Index: cc/output_surface.h
diff --git a/cc/output_surface.h b/cc/output_surface.h
index 4bc31802b23153e7bbfe80aa94660c2d56d43ec7..98f636a53a53ab3be80a4ca9adf0ec7694c234bd 100644
--- a/cc/output_surface.h
+++ b/cc/output_surface.h
@@ -61,6 +61,11 @@ class CC_EXPORT OutputSurface : public WebKit::WebCompositorOutputSurface {
   // Notifies frame-rate smoothness preference. If true, all non-critical
   // processing should be stopped, or lowered in priority.
   virtual void UpdateSmoothnessTakesPriority(bool prefer_smoothness) {}
+
+  // Requests a vsync notification from the output surface. The notification
+  // will be delivered by calling OutputSurfaceClient::DidVSync for all future
+  // vsync events until the callback is disabled.
+  virtual void EnableVSyncNotification(bool enable) {}
 };
 
 }  // namespace cc