chromeos: changes how DisplayManagerObservers are notified

ash/mus/bridge/shell_port_mash.h

Index: ash/mus/bridge/shell_port_mash.h
diff --git a/ash/mus/bridge/shell_port_mash.h b/ash/mus/bridge/shell_port_mash.h
index 2624227b90bccf11525cbf5fb66b773b8d1af4cd..669f6b98cfc7288cf25ecdedb2e5aba4cfd9988e 100644
--- a/ash/mus/bridge/shell_port_mash.h
+++ b/ash/mus/bridge/shell_port_mash.h
@@ -25,6 +25,7 @@ class PointerWatcherEventRouter;
 namespace ash {
 
 class AcceleratorControllerDelegateAura;
+class DisplaySynchronizer;
 class PointerWatcherAdapter;
 class RootWindowController;
 
@@ -160,6 +161,8 @@ class ShellPortMash : public ShellPort, public WindowTreeHostManager::Observer {
 
   std::unique_ptr<SessionStateDelegate> session_state_delegate_;
 
+  std::unique_ptr<DisplaySynchronizer> display_synchronizer_;
+
   bool added_display_observer_ = false;
   base::ObserverList<WmDisplayObserver> display_observers_;