Expose requireAsync method and make it more robust

extensions/common/api/mojo_private.idl

 // Copyright 2015 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.
 
 // The chrome.mojoPrivate API provides access to the mojo modules.
 namespace mojoPrivate {
 
   // |modules| is an array of the values returned by |dependencies| factories.
   callback DefineCallback = void (any[] modules);
 
   interface Functions {
     // Defines a AMD module.
     [nocompile] static void define(
         DOMString moduleName,
         optional DOMString[] dependencies,
         DefineCallback factory);
+
+    // Returns a promise that will resolve to an asynchronously
+    // loaded module.
+    [nocompile] static any requireAsync(DOMString name);
   };
 
 };