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 5e4813f067553ce9e2f803c863c07d9b863996fe..9af5fad5ace1bd78cfbbb43ca034a65e62aa3155 100644
--- a/cc/ipc/display_compositor.mojom
+++ b/cc/ipc/display_compositor.mojom
@@ -67,6 +67,10 @@ interface DisplayCompositor {
   // Removes a BeginFrame relationship between two FrameSinkIds.
   UnregisterFrameSinkHierarchy(cc.mojom.FrameSinkId parent_frame_sink_id,
                                cc.mojom.FrameSinkId child_frame_sink_id);
+
+  // 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