Add/remove surface references via MojoCompositorFrameSink.

services/ui/ws/window_server.cc

Index: services/ui/ws/window_server.cc
diff --git a/services/ui/ws/window_server.cc b/services/ui/ws/window_server.cc
index 381daf2a25c00b8f8c781e6c3cc01c42aa64ed6d..505042f47ff7209b466f72a53fbd59fa53674daa 100644
--- a/services/ui/ws/window_server.cc
+++ b/services/ui/ws/window_server.cc
@@ -525,6 +525,11 @@ cc::mojom::DisplayCompositor* WindowServer::GetDisplayCompositor() {
   return display_compositor_.get();
 }
 
+const cc::SurfaceId& WindowServer::GetRootSurfaceId() const {
+  DCHECK(root_surface_id_.local_frame_id().is_valid());
+  return root_surface_id_;
+}
+
 bool WindowServer::GetFrameDecorationsForUser(
     const UserId& user_id,
     mojom::FrameDecorationValuesPtr* values) {
@@ -804,6 +809,11 @@ void WindowServer::OnSurfaceCreated(const cc::SurfaceId& surface_id,
   }
 }
 
+void WindowServer::OnDisplayCompositorCreated(
+    const cc::SurfaceId& root_surface_id) {
+  root_surface_id_ = root_surface_id;
+}
+
 void WindowServer::OnActiveUserIdChanged(const UserId& previously_active_id,
                                          const UserId& active_id) {
 }