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 794f258a32f92758fcf0eab22311cbb54257a581..c611080de2d6b621ac4ea2cefc0424dd4f011af9 100644
--- a/services/ui/ws/window_server.cc
+++ b/services/ui/ws/window_server.cc
@@ -515,6 +515,11 @@ cc::mojom::DisplayCompositor* WindowServer::GetDisplayCompositor() {
   return display_compositor_.get();
 }
 
+const cc::SurfaceId& WindowServer::GetRootSurfaceId() const {
+  DCHECK(!root_surface_id_.local_frame_id().nonce().is_empty());
+  return root_surface_id_;
+}
+
 bool WindowServer::GetFrameDecorationsForUser(
     const UserId& user_id,
     mojom::FrameDecorationValuesPtr* values) {
@@ -794,6 +799,11 @@ void WindowServer::OnSurfaceCreated(const cc::SurfaceId& surface_id,
   }
 }
 
+void WindowServer::OnRootSurfaceId(const cc::SurfaceId& root_surface_id) {
+  LOG(ERROR) << "Got root surface id " << root_surface_id;
+  root_surface_id_ = root_surface_id;
+}
+
 void WindowServer::OnActiveUserIdChanged(const UserId& previously_active_id,
                                          const UserId& active_id) {
 }