Fix warnings and errors in ChromeVox since Closure compiler update.

chrome/browser/resources/chromeos/chromevox/common/externs.js

 // 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.
 
 Range.prototype.getBoundingClientRect;
 Document.prototype.documentElement.innerWidth;
 Document.prototype.documentElement.head;
-
-/** @constructor */
-function WeakMap() {}
-
-/**
- * @param {Object} key
- * @return {*}
- */
-WeakMap.prototype.get = function(key) {};
-
-/**
- * @param {Object} key
- * @param {*} value
- */
-WeakMap.prototype.set = function(key, value) {};