Extend the capability of SystemMonitor to support watching storage free space change

chrome/browser/system_monitor/media_device_notifications_chromeos.h

Index: chrome/browser/system_monitor/media_device_notifications_chromeos.h
diff --git a/chrome/browser/system_monitor/media_device_notifications_chromeos.h b/chrome/browser/system_monitor/media_device_notifications_chromeos.h
index c0672d14b6c8008aa1ae09a82fd63655e4d71d9d..dc931027ac36c269983762e316270ca3ba9fea3e 100644
--- a/chrome/browser/system_monitor/media_device_notifications_chromeos.h
+++ b/chrome/browser/system_monitor/media_device_notifications_chromeos.h
@@ -25,7 +25,8 @@ namespace chromeos {
 
 class MediaDeviceNotifications
     : public base::RefCountedThreadSafe<MediaDeviceNotifications>,
-      public disks::DiskMountManager::Observer {
+      public disks::DiskMountManager::Observer,
+      public base::SystemMonitor::StorageFreeSpaceNotification {
  public:
   MediaDeviceNotifications();
 
@@ -61,6 +62,10 @@ class MediaDeviceNotifications
   void AddMountedPathOnUIThread(
       const disks::DiskMountManager::MountPointInfo& mount_info);
 
+  // Overriden from SystemMonitor::StorageFreeSpaceNotification.
+  virtual bool StartWatchingStorage(const FilePath::StringType& path) OVERRIDE;
+  virtual bool StopWatchingStorage(const FilePath::StringType& path) OVERRIDE;
+
   // Mapping of relevant mount points and their corresponding mount devices.
   // Only accessed on the UI thread.
   MountMap mount_map_;