mash: Move //ash/session to //ash/common/session

ash/session/session_state_observer.cc

-// Copyright 2013 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 "ash/session/session_state_observer.h"
-
-#include "ash/session/session_state_delegate.h"
-#include "ash/shell.h"
-
-namespace ash {
-
-ScopedSessionStateObserver::ScopedSessionStateObserver(
-    ash::SessionStateObserver* observer)
-    : observer_(observer) {
-  ash::Shell::GetInstance()->session_state_delegate()->AddSessionStateObserver(
-      observer_);
-}
-
-ScopedSessionStateObserver::~ScopedSessionStateObserver() {
-  ash::Shell::GetInstance()
-      ->session_state_delegate()
-      ->RemoveSessionStateObserver(observer_);
-}
-
-}  // namespace ash