Moves ShellObserver to ash/common

ash/system/cast/tray_cast.h

 // 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.
 
 #ifndef ASH_SYSTEM_CAST_TRAY_CAST_H_
 #define ASH_SYSTEM_CAST_TRAY_CAST_H_
 
 #include "ash/cast_config_delegate.h"
-#include "ash/shell_observer.h"
+#include "ash/common/shell_observer.h"
 #include "ash/system/tray/system_tray_item.h"
 #include "base/macros.h"
 
 namespace ash {
 namespace tray {
 class CastTrayView;
 class CastSelectDefaultView;
 class CastDetailedView;
 class CastDuplexView;
 }  // namespace tray
@@ skipping 51 matching lines @@
   tray::CastTrayView* tray_ = nullptr;
   tray::CastDuplexView* default_ = nullptr;
   tray::CastDetailedView* detailed_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(TrayCast);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_CAST_TRAY_CAST_H_