SystemMonitor refactoring: move power state monitor into a separate class called PowerMonitor

base/system_monitor/system_monitor_ios.mm

-// 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.
-
-#include "base/system_monitor/system_monitor.h"
-
-#import <UIKit/UIKit.h>
-
-namespace base {
-
-void SystemMonitor::PlatformInit() {
-  NSNotificationCenter* nc = [NSNotificationCenter defaultCenter];
-  id foreground =
-      [nc addObserverForName:UIApplicationWillEnterForegroundNotification
-                      object:nil
-                       queue:nil
-                  usingBlock:^(NSNotification* notification) {
-                      ProcessPowerMessage(RESUME_EVENT);
-                  }];
-  id background =
-      [nc addObserverForName:UIApplicationDidEnterBackgroundNotification
-                      object:nil
-                       queue:nil
-                  usingBlock:^(NSNotification* notification) {
-                      ProcessPowerMessage(SUSPEND_EVENT);
-                  }];
-  notification_observers_.push_back(foreground);
-  notification_observers_.push_back(background);
-}
-
-void SystemMonitor::PlatformDestroy() {
-  NSNotificationCenter* nc = [NSNotificationCenter defaultCenter];
-  for (std::vector<id>::iterator it = notification_observers_.begin();
-       it != notification_observers_.end(); ++it) {
-    [nc removeObserver:*it];
-  }
-  notification_observers_.clear();
-}
-
-}  // namespace base