Remove enable Drive offline system tray UI.

ash/system/drive/drive_observer.h

 // Copyright (c) 2012 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_DRIVE_DRIVE_OBSERVER_H_
 #define ASH_SYSTEM_DRIVE_DRIVE_OBSERVER_H_
 
 #include "ash/system/tray/system_tray_delegate.h"
 
 namespace ash {
 
 class DriveObserver {
  public:
   virtual void OnDriveJobUpdated(const DriveOperationStatus& status) = 0;
-  virtual void OnDriveOfflineEnabled() = 0;
 
  protected:
   virtual ~DriveObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_DRIVE_DRIVE_OBSERVER_H_