mus-ws: Plumb FrameSinkId to Children

ui/aura/mus/mus_context_factory.h

Index: ui/aura/mus/mus_context_factory.h
diff --git a/ui/aura/mus/mus_context_factory.h b/ui/aura/mus/mus_context_factory.h
index babca4790a3998713dfb172dc30581dd5b699340..0af72773632f73f309c08d4aff70c345f3ff3049 100644
--- a/ui/aura/mus/mus_context_factory.h
+++ b/ui/aura/mus/mus_context_factory.h
@@ -37,6 +37,10 @@ class AURA_EXPORT MusContextFactory : public ui::ContextFactory {
   void OnEstablishedGpuChannel(base::WeakPtr<ui::Compositor> compositor,
                                scoped_refptr<gpu::GpuChannelHost> gpu_channel);
 
+  void OnCompositorFrameSinkAvailable(
+      base::WeakPtr<ui::Compositor> compositor,
+      std::unique_ptr<cc::CompositorFrameSink> compositor_frame_sink);
+
   // ContextFactory:
   void CreateCompositorFrameSink(
       base::WeakPtr<ui::Compositor> compositor) override;