Unify mustash display closing code paths.

services/ui/display/platform_screen.h

Index: services/ui/display/platform_screen.h
diff --git a/services/ui/display/platform_screen.h b/services/ui/display/platform_screen.h
index a347a9718a2db4158396860e3e4ce9932c52e2e0..f99265611c966e4fe35e0d544c8524fe9376665d 100644
--- a/services/ui/display/platform_screen.h
+++ b/services/ui/display/platform_screen.h
@@ -37,6 +37,9 @@ class PlatformScreen {
   // removed or modified.
   virtual void Init(PlatformScreenDelegate* delegate) = 0;
 
+  // Handle requests from the platform to close a display.
+  virtual void RequestCloseDisplay(int64_t display_id) = 0;
+
   virtual int64_t GetPrimaryDisplayId() const = 0;
 
  private: