Don't dispatch event after shutdown on AshWTHUnified

ash/host/ash_window_tree_host_unified.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/host/ash_window_tree_host_unified.h"
 #include "ash/host/root_window_transformer.h"
 #include "base/logging.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/window_targeter.h"
 #include "ui/compositor/compositor.h"
 #include "ui/events/event_processor.h"
+#include "ui/events/null_event_targeter.h"
 #include "ui/gfx/geometry/insets.h"
 
 namespace ash {
 
 class UnifiedEventTargeter : public aura::WindowTargeter {
  public:
   UnifiedEventTargeter(aura::Window* src_root, aura::Window* dst_root)
       : src_root_(src_root), dst_root_(dst_root) {}
 
   ui::EventTarget* FindTargetForEvent(ui::EventTarget* root,
@@ skipping 26 matching lines @@
     : bounds_(gfx::Rect(initial_bounds.size())) {
   CreateCompositor(GetAcceleratedWidget());
 }
 
 AshWindowTreeHostUnified::~AshWindowTreeHostUnified() {
   DestroyCompositor();
   DestroyDispatcher();
 }
 
 void AshWindowTreeHostUnified::PrepareForShutdown() {
+  window()->SetEventTargeter(
+      scoped_ptr<ui::EventTargeter>(new ui::NullEventTargeter));
+
   for (auto host : mirroring_hosts_)
     host->PrepareForShutdown();
 }
 
 void AshWindowTreeHostUnified::RegisterMirroringHost(
     AshWindowTreeHost* mirroring_ash_host) {
   aura::Window* src_root = mirroring_ash_host->AsWindowTreeHost()->window();
   src_root->SetEventTargeter(
       make_scoped_ptr(new UnifiedEventTargeter(src_root, window())));
   DCHECK(std::find(mirroring_hosts_.begin(), mirroring_hosts_.end(),
@@ skipping 86 matching lines @@
   DCHECK(iter != mirroring_hosts_.end());
   window->RemoveObserver(this);
   mirroring_hosts_.erase(iter);
 }
 
 ui::EventProcessor* AshWindowTreeHostUnified::GetEventProcessor() {
   return dispatcher();
 }
 
 }  // namespace ash