Revert "chromeos: Move files in //ash/common to //ash, part 3"

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/wm_shell.h"
-
-namespace ash {
-
-ScopedSessionStateObserver::ScopedSessionStateObserver(
-    SessionStateObserver* observer)
-    : observer_(observer) {
-  WmShell::Get()->GetSessionStateDelegate()->AddSessionStateObserver(observer_);
-}
-
-ScopedSessionStateObserver::~ScopedSessionStateObserver() {
-  if (WmShell::Get()) {
-    WmShell::Get()->GetSessionStateDelegate()->RemoveSessionStateObserver(
-        observer_);
-  }
-}
-
-}  // namespace ash