Moves ShellObserver to ash/common

ash/system/overview/overview_button_tray.h

 // Copyright 2014 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_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_
 #define ASH_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_
 
 #include "ash/ash_export.h"
 #include "ash/common/session/session_state_observer.h"
-#include "ash/shell_observer.h"
+#include "ash/common/shell_observer.h"
 #include "ash/system/tray/tray_background_view.h"
 #include "base/macros.h"
 
 namespace views {
 class ImageView;
 }
 
 namespace ash {
 
 // Status area tray for showing a toggle for Overview Mode. Overview Mode
@@ skipping 42 matching lines @@
 
   // Weak pointer, will be parented by TrayContainer for its lifetime.
   views::ImageView* icon_;
 
   DISALLOW_COPY_AND_ASSIGN(OverviewButtonTray);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_OVERVIEW_OVERVIEW_BUTTON_TRAY_H_