WIP: Propagate SurfaceID up window tree hierarchy

services/ui/public/cpp/window_observer.h

Index: services/ui/public/cpp/window_observer.h
diff --git a/services/ui/public/cpp/window_observer.h b/services/ui/public/cpp/window_observer.h
index 83ed271c2cfeecefe10268c5be4fa85ae4f26641..af7b4b817cebd4644da65917032ffb4edb16e887 100644
--- a/services/ui/public/cpp/window_observer.h
+++ b/services/ui/public/cpp/window_observer.h
@@ -48,6 +48,13 @@ class WindowObserver {
   virtual void OnWindowDestroying(Window* window) {}
   virtual void OnWindowDestroyed(Window* window) {}
 
+  virtual void OnChildWindowSurfaceCreated(
+      Window* window,
+      const gfx::Size& size,
+      float device_scale_factor,
+      const cc::SurfaceId& surface_id,
+      const cc::SurfaceSequence& surface_sequence) {}
+
   virtual void OnWindowBoundsChanging(Window* window,
                                       const gfx::Rect& old_bounds,
                                       const gfx::Rect& new_bounds) {}