Allow switching between extended and unified

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"
@@ skipping 35 matching lines @@
 };
 
 AshWindowTreeHostUnified::AshWindowTreeHostUnified(
     const gfx::Rect& initial_bounds)
     : bounds_(gfx::Rect(initial_bounds.size())), transformer_helper_(this) {
   CreateCompositor(GetAcceleratedWidget());
   transformer_helper_.Init();
 }
 
 AshWindowTreeHostUnified::~AshWindowTreeHostUnified() {
+  for (auto* ash_host : mirroring_hosts_)
+    ash_host->AsWindowTreeHost()->window()->RemoveObserver(this);
+
   DestroyCompositor();
   DestroyDispatcher();
 }
 
 void AshWindowTreeHostUnified::PrepareForShutdown() {
   window()->SetEventTargeter(
       scoped_ptr<ui::EventTargeter>(new ui::NullEventTargeter));
 
   for (auto host : mirroring_hosts_)
     host->PrepareForShutdown();
@@ skipping 110 matching lines @@
   DCHECK(iter != mirroring_hosts_.end());
   window->RemoveObserver(this);
   mirroring_hosts_.erase(iter);
 }
 
 ui::EventProcessor* AshWindowTreeHostUnified::GetEventProcessor() {
   return dispatcher();
 }
 
 }  // namespace ash