mus: Make sure WindowCompositorFrameSink is detached correctly.

services/ui/public/cpp/window_compositor_frame_sink.cc

Index: services/ui/public/cpp/window_compositor_frame_sink.cc
diff --git a/services/ui/public/cpp/window_compositor_frame_sink.cc b/services/ui/public/cpp/window_compositor_frame_sink.cc
index 130e6d5d0f2b088823d20b6d53838bc4b93b0c0f..4b3f6759d5cc2f53ff9997e49280600d96613442 100644
--- a/services/ui/public/cpp/window_compositor_frame_sink.cc
+++ b/services/ui/public/cpp/window_compositor_frame_sink.cc
@@ -56,6 +56,7 @@ bool WindowCompositorFrameSink::BindToClient(
 void WindowCompositorFrameSink::DetachFromClient() {
   client_->SetBeginFrameSource(nullptr);
   begin_frame_source_.reset();
+  client_binding_.reset();
   compositor_frame_sink_.reset();
   cc::CompositorFrameSink::DetachFromClient();
 }