Add/remove surface references via MojoCompositorFrameSink.

services/ui/ws/server_window_delegate.h

Index: services/ui/ws/server_window_delegate.h
diff --git a/services/ui/ws/server_window_delegate.h b/services/ui/ws/server_window_delegate.h
index 08db68fc1ded808bc7cac01f3f531d033a92c284..a6e41d586580693ad60b0b04bde1d07deae85c96 100644
--- a/services/ui/ws/server_window_delegate.h
+++ b/services/ui/ws/server_window_delegate.h
@@ -31,6 +31,8 @@ class ServerWindowDelegate {
   // DisplayCompositor running in the system.
   virtual cc::mojom::DisplayCompositor* GetDisplayCompositor() = 0;
 
+  virtual const cc::SurfaceId& GetRootSurfaceId() const = 0;
+
   // Returns the root of the window tree to which this |window| is attached.
   // Returns null if this window is not attached up through to a root window.
   virtual ServerWindow* GetRootWindow(const ServerWindow* window) = 0;