Set limit on maximum width of ChromeVox menus.

chrome/browser/resources/chromeos/chromevox/cvox2/background/panel.css

 /* Copyright 2016 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. */
 
 body {
   border: 0;
   height: 35px;
   margin: 0;
   overflow: hidden;
   padding: 0;
@@ skipping 215 matching lines @@
 .menu-item:hover,
 .menu-item.hover,
 .menu-item:focus {
   background-color: rgb(246, 145, 57);
   color: #fff;
 }
 
 .menu-item-title {
   border: 0;
   margin: 0;
+  max-width: 500px;
+  overflow: hidden;
   padding: 0 18px;
+  text-overflow: ellipsis;
+  white-space: nowrap;
 }
 
 .menu-item-shortcut {
   border: 0;
   margin: 0;
   padding: 0 18px 0 24px;
   text-align: right;
 }
 
 #tutorial {
@@ skipping 86 matching lines @@
   border: 1px solid black;
 }
 
 /*
  * The html hidden attribute doesn't make elements not focusable by default.
  * This rule ensures that elements that are hidden truly don't show up.
  */
 *[hidden] {
   display: none !important;
 }