Adds missing USB events to the imageWriterPrivate API definition.

chrome/common/extensions/api/image_writer_private.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.image_writer</code> API to write images to
 // removable media.
 [nodoc] namespace imageWriterPrivate {
   // The different stages of a write call.
   //
   // <dl>
@@ skipping 101 matching lines @@
     // List all the removable block devices currently attached to the system.
     // |callback|: A callback called with a list of removable storage devices
     static void listRemovableStorageDevices(
         ListRemovableStorageDevicesCallback callback);
   };
 
   interface Events {
     // Fires periodically throughout the writing operation and at least once per
     // stage.
     static void onWriteProgress(ProgressInfo info);
+
     // Fires when the write operation has completely finished, such as all
     // devices being finalized and resources released.
     static void onWriteComplete();
+
     // Fires when an error occured during writing, passing the $ref:ProgressInfo
     // of the operation at the time the error occured.
     static void onWriteError(ProgressInfo info, DOMString error);
+
+    // Fires when a removable storage device is inserted.
+    static void onDeviceInserted(RemovableStorageDevice device);
+
+    // Fires when a removable storage device is removed.
+    static void onDeviceRemoved(RemovableStorageDevice device);
   };
 
 };