Add the chrome.fileSystem.onVolumeListChanged event.

chrome/common/extensions/api/file_system.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.
 
 // Use the <code>chrome.fileSystem</code> API to create, read, navigate,
 // and write to the user's local file system. With this API, Chrome Apps can
 // read and write to a user-selected location. For example, a text editor app
 // can use the API to read and write local documents. All failures are notified
 // via chrome.runtime.lastError.
 namespace fileSystem {
@@ skipping 87 matching lines @@
     DOMString volumeId;
     boolean writable;
   };
 
   // Change to an entry within a tracked directory.
   [nodoc] dictionary ChildChange {
     [instanceOf=Entry] object entry;
     ChildChangeType type;
   };
 
+  // Event notifying about an inserted or a removed volume from the system.
+  dictionary VolumeListChangedEvent {
+    Volume[] volumes;
+  };
+
   // Event notifying about a change in a file or a directory, including its
   // contents.
   [nodoc] dictionary EntryChangedEvent {
     // Tracked entry.
     [instanceOf=Entry] object target;
 
     // List of changed entries within the tracked directory in order they
     // happened. May not be available for some types of file systems.
     ChildChange[]? childChanges;
   };
@@ skipping 92 matching lines @@
         optional boolean recursive);
 
     // Unobserves a previously observed either a file or a directory.
     [nodoc] static void unobserveEntry([instanceOf=Entry] object entry);
 
     // Lists all observed entries.
     [nodoc] static void getObservedEntries(GetObservedEntriesCallback callback);
   };
 
   interface Events {
+    // Called when a list of available volumes is changed.
+    static void onVolumeListChanged(VolumeListChangedEvent event);
+
     // Called when an observed entry is changed.
     [nodoc] static void onEntryChanged(EntryChangedEvent event);
 
     // Called when an observed entry is removed.
     [nodoc] static void onEntryRemoved(EntryRemovedEvent event);
   };
 };