Support registering and dispatching passive event listeners.

third_party/WebKit/Source/core/events/EventListenerOptions.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.
 
 // https://github.com/RByers/EventListenerOptions
 
 dictionary EventListenerOptions {
     // TODO: |capture| is runtime enabled and its value actually
     // changes whether the feature is enabled or not. Setting
     // it to a default value here prevents the ability of checking
     // whether it was set manually or by default. See EventTarget.cpp
     // where it is populated. crbug.com/543685
     [RuntimeEnabled=EventListenerOptions] boolean capture;
+    [RuntimeEnabled=PassiveEventListeners] boolean passive;
 };