Remove dispatcher when shutting down RootWindowController

ash/host/ash_window_tree_host_x11.cc

Index: ash/host/ash_window_tree_host_x11.cc
diff --git a/ash/host/ash_window_tree_host_x11.cc b/ash/host/ash_window_tree_host_x11.cc
index eff9ba60f9e98139d549da3e2d9a9830b4435ff0..beb129ff97a724038733f1a0e8734ee9d87525eb 100644
--- a/ash/host/ash_window_tree_host_x11.cc
+++ b/ash/host/ash_window_tree_host_x11.cc
@@ -19,9 +19,11 @@
 #include "ui/aura/client/screen_position_client.h"
 #include "ui/aura/env.h"
 #include "ui/aura/window.h"
+#include "ui/aura/window_event_dispatcher.h"
 #include "ui/base/x/x11_util.h"
 #include "ui/events/event.h"
 #include "ui/events/event_utils.h"
+#include "ui/events/platform/platform_event_source.h"
 #include "ui/events/x/device_data_manager.h"
 #include "ui/events/x/device_list_cache_x.h"
 #include "ui/events/x/touch_factory_x11.h"
@@ -129,6 +131,11 @@ void AshWindowTreeHostX11::UpdateDisplayID(int64 id1, int64 id2) {
   display_ids_.second = id2;
 }
 
+void AshWindowTreeHostX11::PrepareForShutdown() {
+  if (ui::PlatformEventSource::GetInstance())
+    ui::PlatformEventSource::GetInstance()->RemovePlatformEventDispatcher(this);
+}
+
 void AshWindowTreeHostX11::SetBounds(const gfx::Rect& bounds) {
   WindowTreeHostX11::SetBounds(bounds);
   if (pointer_barriers_) {