Moves ShellObserver to ash/common

ash/aura/wm_shell_aura.h

 // Copyright 2016 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.
 
 #ifndef ASH_AURA_WM_SHELL_AURA_H_
 #define ASH_AURA_WM_SHELL_AURA_H_
 
 #include <set>
 
 #include "ash/ash_export.h"
 #include "ash/aura/wm_lookup_aura.h"
+#include "ash/common/shell_observer.h"
 #include "ash/common/wm_shell.h"
 #include "ash/display/window_tree_host_manager.h"
-#include "ash/shell_observer.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "ui/wm/public/activation_change_observer.h"
 
 namespace ash {
 
 class ASH_EXPORT WmShellAura : public WmShell,
                                public aura::client::ActivationChangeObserver,
                                public WindowTreeHostManager::Observer,
                                public ShellObserver {
@@ skipping 54 matching lines @@
   base::ObserverList<WmDisplayObserver> display_observers_;
 
   base::ObserverList<WmOverviewModeObserver> overview_mode_observers_;
 
   DISALLOW_COPY_AND_ASSIGN(WmShellAura);
 };
 
 }  // namespace ash
 
 #endif  // ASH_AURA_WM_SHELL_AURA_H_