[SystemInfo API] Implement storage.getAvailableCapacity API

chrome/common/extensions/api/system_storage.idl

 // 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.
 
 // Use the <code>chrome.system.storage</code> API to query storage device
 // information and be notified when a removable storage device is attached and
 // detached.
 namespace system.storage {
 
   enum StorageUnitType {
@@ skipping 12 matching lines @@
     // application, or between different applications.
     DOMString id;
     // The name of the storage unit.
     DOMString name;
     // The media type of the storage unit.
     StorageUnitType type;
     // The total amount of the storage space, in bytes.
     double capacity;
   };
 
+  dictionary StorageAvailableCapacityInfo {
+    // A copied |id| of getAvailableCapacity function parameter |id|.
+    DOMString id;
+    // The available capacity of the storage device, in bytes.
+    double availableCapacity;
+  };
+
   [inline_doc] enum EjectDeviceResultCode {
     // The ejection command is successful -- the application can prompt the user
     // to remove the device.
     success,
     // The device is in use by another application. The ejection did not
     // succeed; the user should not remove the device until the other
     // application is done with the device.
     in_use,
     // There is no such device known.
     no_such_device,
     // The ejection command failed.
     failure
   };
 
   callback EjectDeviceCallback = void (EjectDeviceResultCode result);
 
   callback StorageInfoCallback = void (StorageUnitInfo[] info);
 
+  callback GetAvailableCapacityCallback = void (
+      StorageAvailableCapacityInfo info);
+
   interface Functions {
     // Get the storage information from the system. The argument passed to the
     // callback is an array of StorageUnitInfo objects.
     static void getInfo(StorageInfoCallback callback);
 
     // Ejects a removable storage device.
     static void ejectDevice(DOMString id, EjectDeviceCallback callback);
+
+    // Get the available capacity of a specified |id| storage device.
+    // The |id| is the transient device ID from StorageUnitInfo.
+    static void getAvailableCapacity(DOMString id,
+                                     GetAvailableCapacityCallback callback);
   };
 
   interface Events {
     // Fired when a new removable storage is attached to the system.
     static void onAttached(StorageUnitInfo info);
 
     // Fired when a removable storage is detached from the system.
     static void onDetached(DOMString id);
   };
 
 };
-