Fix AshWindowTreeHost shutdown for Ozone X11.

ui/platform_window/platform_window.h

Index: ui/platform_window/platform_window.h
diff --git a/ui/platform_window/platform_window.h b/ui/platform_window/platform_window.h
index f22a48cd50bf21bc5243838086417a36cce51da5..bd8dbb01ef9c3dc6fbee728aea6491fa67a0e95e 100644
--- a/ui/platform_window/platform_window.h
+++ b/ui/platform_window/platform_window.h
@@ -30,6 +30,11 @@ class PlatformWindow {
   virtual void Hide() = 0;
   virtual void Close() = 0;
 
+  // Informs the window it is going to be destroyed sometime soon. This is only
+  // called for specific code paths, for example by Ash, so it shouldn't be
+  // assumed this will get called before destruction.
+  virtual void PrepareForShutdown() = 0;
+
   // Sets and gets the bounds of the platform-window. Note that the bounds is in
   // physical pixel coordinates.
   virtual void SetBounds(const gfx::Rect& bounds) = 0;