Unify mustash display closing code paths.

services/ui/ws/platform_display_delegate.h

Index: services/ui/ws/platform_display_delegate.h
diff --git a/services/ui/ws/platform_display_delegate.h b/services/ui/ws/platform_display_delegate.h
index 9c92f58dd7d299aef9657462bd2d9d6adbcc076a..7daf17da384388c6c2d3dfd05f2b95349ece3cfb 100644
--- a/services/ui/ws/platform_display_delegate.h
+++ b/services/ui/ws/platform_display_delegate.h
@@ -28,9 +28,6 @@ class PlatformDisplayDelegate {
 
   virtual bool IsInHighContrastMode() = 0;
 
-  // Called when the window managed by the PlatformDisplay is closed.
-  virtual void OnDisplayClosed() = 0;
-
   // Called when an event arrives.
   virtual void OnEvent(const ui::Event& event) = 0;