Fix AshWindowTreeHost shutdown for Ozone X11.

ash/host/ash_window_tree_host_platform.cc

Index: ash/host/ash_window_tree_host_platform.cc
diff --git a/ash/host/ash_window_tree_host_platform.cc b/ash/host/ash_window_tree_host_platform.cc
index a268db387a45ff9e24835916bbf2a8df038e468e..e52814565fa78023387d8d87efbf620acc7b1f0b 100644
--- a/ash/host/ash_window_tree_host_platform.cc
+++ b/ash/host/ash_window_tree_host_platform.cc
@@ -75,6 +75,10 @@ void AshWindowTreeHostPlatform::PrepareForShutdown() {
   // coordinates.
   window()->SetEventTargeter(
       std::unique_ptr<ui::EventTargeter>(new ui::NullEventTargeter));
+
+  // Do anything platform specific necessary before shutdown (eg. stop
+  // listening for configuration XEvents).
+  platform_window()->PrepareForShutdown();
 }
 
 void AshWindowTreeHostPlatform::SetRootTransform(