Make callback optional.

chrome/common/extensions/api/browser.idl

 // Copyright 2014 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.browser</code> API to interact with the Chrome browser
 // associated with the current application and Chrome profile.
 namespace browser {
   // Options for the $(ref:openTab) function.
   dictionary OpenTabOptions {
     // The URL to navigate to when the new tab is initially opened.
     DOMString url;
   };
 
   callback Callback = void();
 
   interface Functions {
     // Opens a new tab in a browser window associated with the current
     // application and Chrome profile. If no browser window for the Chrome
     // profile is opened, a new one is opened prior to creating the new tab. The
     // initial URL of the new tab is specified in |options|.
     // |options|  : The $(ref:OpenTabOptions) for this function.
     // |callback| : Called to indicate success or failure.
     static void openTab(OpenTabOptions options,
-                        Callback callback);
+                        optional Callback callback);
   };
 };