mash: Avoid shelf crashes on display removal.

ash/mus/root_window_controller.h

Index: ash/mus/root_window_controller.h
diff --git a/ash/mus/root_window_controller.h b/ash/mus/root_window_controller.h
index 5f467a8c1e3503fe506b2be418c8ef43e9f98ac3..fe4e3e3226da0993c590dd0f2b9a1ed3b41e84e0 100644
--- a/ash/mus/root_window_controller.h
+++ b/ash/mus/root_window_controller.h
@@ -12,6 +12,7 @@
 #include "services/ui/public/cpp/window_observer.h"
 #include "services/ui/public/interfaces/window_manager_constants.mojom.h"
 #include "ui/display/display.h"
+#include "ui/display/display_observer.h"
 
 namespace shell {
 class Connector;
@@ -68,6 +69,8 @@ class RootWindowController {
 
   WmShelfMus* wm_shelf() { return wm_shelf_.get(); }
 
+  void Shutdown();
+
  private:
   friend class WmTestBase;
   friend class WmTestHelper;