Add the basic code skeleton for system info event router

chrome/common/extensions/api/experimental_system_info_storage.idl

 // 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.
 
 namespace experimental.systemInfo.storage {
 
   enum StorageUnitType {
     // Unknow storage type.
     unknown,
     // Hard disk.
     harddisk,
     // Removable storage, e.g.USB device.
     removable
   };
 
   dictionary StorageUnitInfo {
     // The unique id of the storage unit.
     DOMString id;
     // The type of storage device. The value is one of the constants defined
     // for this type.
     StorageUnitType type;
     // The total amount of the storage space, in bytes.
     double capacity;
     // The available amount of the storage space, in bytes.
     double availableCapacity;
   };
 
+  dictionary StorageChangeInfo {
+    // The uniue id of the storage unit already changed.
+    DOMString id;
+    // The new value of the available capacity.
+    double availableCapacity;
+  };
+
   callback StorageInfoCallback = void (StorageUnitInfo[] info);
 
   interface Functions {
-    // Get the storage information from the system.
+    // Get the storage information from the system. The argument passed to the
+    // callback is an array of StorageUnitInfo objects.
     static void get(StorageInfoCallback callback);
   };
+
+  interface Events {
+    // Fired when the storage information is updated.
+    // |info|   : The changed information for the specified storage unit
+    static void onAvailableCapacityChanged(StorageChangeInfo info);
+  };
+ 
 };