Add WorkspaceObserver to observe changes in workspace state

views/widget/native_widget_aura.cc

 // Copyright (c) 2011 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 "views/widget/native_widget_aura.h"
 
 #include "base/bind.h"
 #include "ui/aura/aura_constants.h"
 #include "ui/aura/desktop.h"
 #include "ui/aura/desktop_observer.h"
@@ skipping 598 matching lines @@
 
 void NativeWidgetAura::OnWindowDestroying() {
   delegate_->OnNativeWidgetDestroying();
 
   // If the aura::Window is destroyed, we can no longer show tooltips.
   tooltip_manager_.reset();
 }
 
 void NativeWidgetAura::OnWindowDestroyed() {
   window_ = NULL;
+  tooltip_manager_.reset();
   delegate_->OnNativeWidgetDestroyed();
   if (ownership_ == Widget::InitParams::NATIVE_WIDGET_OWNS_WIDGET)
     delete this;
 }
 
 void NativeWidgetAura::OnWindowVisibilityChanged(bool visible) {
   delegate_->OnNativeWidgetVisibilityChanged(visible);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 82 matching lines @@
 }
 
 // static
 bool NativeWidgetPrivate::IsMouseButtonDown() {
   NOTIMPLEMENTED();
   return false;
 }
 
 }  // namespace internal
 }  // namespace views