Implement temporary reference assignment with DisplayCompositor.

cc/ipc/display_compositor.mojom

Index: cc/ipc/display_compositor.mojom
diff --git a/cc/ipc/display_compositor.mojom b/cc/ipc/display_compositor.mojom
index 52b688aafd62cd930c54aa99de03ad63e015d645..0ae567ae6cd23b4f775625cdd47f1306d7282963 100644
--- a/cc/ipc/display_compositor.mojom
+++ b/cc/ipc/display_compositor.mojom
@@ -57,6 +57,10 @@ interface DisplayCompositor {
       cc.mojom.MojoCompositorFrameSink& compositor_frame_sink,
       cc.mojom.MojoCompositorFrameSinkPrivate& compositor_frame_sink_private,
       cc.mojom.MojoCompositorFrameSinkClient compositor_frame_sink_client);
+
+  // Drops the temporary reference for |surface_id|. This will get called when
+  // the DisplayCompositorClient doesn't think |surface_id| will be embedded.
+  DropTemporaryReference(cc.mojom.SurfaceId surface_id);
 };
 
 // The DisplayCompositorClient interface is implemented by the Display