Add a note that user gesture is required to display the media galleries dialog.

chrome/common/extensions/api/media_galleries.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 mediaGalleries {
 
   [inline_doc] enum GetMediaFileSystemsInteractivity {
     // Do not act interactively.
     no,
     // Ask the user to manage permitted media galleries.
     yes,
     // Ask the user to manage permitted galleries only if the return set would
     // otherwise be empty.
     if_needed
   };
 
   [inline_doc] dictionary MediaFileSystemsDetails {
     // Whether to prompt the user for permission to additional media galleries
     // before returning the permitted set. Default is silent.  If the value
     // 'yes' is passed, or if the application has not been granted access to
     // any media galleries and the value 'if_needed' is passed, then the
-    // media gallery configuration dialog will be displayed.
+    // media gallery configuration dialog will be displayed. A user
+    // gesture is required for the display of the dialog. (i.e. making the call
+    // in the context of a button press)
     GetMediaFileSystemsInteractivity? interactive;
   };
 
   callback MediaFileSystemsCallback =
       void ([instanceOf=DOMFileSystem] optional object[] mediaFileSystems);
 
   [inline_doc] dictionary MediaFileSystemMetadata {
     // The name of the file system.
     DOMString name;
 
@@ skipping 17 matching lines @@
     // configured or available, the callback will receive an empty array.
     static void getMediaFileSystems(optional MediaFileSystemsDetails details,
                                     MediaFileSystemsCallback callback);
 
     // Get metadata about a specific media file system.
     [nocompile] static MediaFileSystemMetadata getMediaFileSystemMetadata(
         [instanceOf=DOMFileSystem] object mediaFileSystem);
   };
 
 };