Improve braille related message descriptions and clean up message handling in Chromevox.

chrome/browser/resources/chromeos/chromevox/walkers/sentence_walker.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.
 
 /**
  * @fileoverview A JavaScript class for walking sentences.
  */
 
 
 goog.provide('cvox.SentenceWalker');
 
 goog.require('cvox.AbstractSelectionWalker');
 goog.require('cvox.TraverseContent');
 
 /**
  * @constructor
  * @extends {cvox.AbstractSelectionWalker}
  */
 cvox.SentenceWalker = function() {
   cvox.AbstractSelectionWalker.call(this);
   this.grain = cvox.TraverseContent.kSentence;
 };
 goog.inherits(cvox.SentenceWalker, cvox.AbstractSelectionWalker);
 
 /**
  * @override
  */
 cvox.SentenceWalker.prototype.getGranularityMsg = function() {
-  return cvox.ChromeVox.msgs.getMsg('sentence_granularity');
+  return Msgs.getMsg('sentence_granularity');
 };