Add ChromeVox panel and implement caption display functionality.

chrome/browser/resources/chromeos/chromevox/chromevox_assets.gypi

 # 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.
 
 {
   'variables': {
     'chromevox_assets_chromevox': [
       'chromevox/chromevox-128.png',
       'chromevox/chromevox-16.png',
       'chromevox/chromevox-19.png',
       'chromevox/chromevox-48.png',
+      'chromevox/close-white.png',
+      'chromevox/close-white-hover.png',
+      'chromevox/options.png',
+      'chromevox/triangle.png',
     ],
     'chromevox_assets_chromevox_background_earcons': [
       'chromevox/background/earcons/alert_modal.ogg',
       'chromevox/background/earcons/alert_nonmodal.ogg',
       'chromevox/background/earcons/bullet.ogg',
       'chromevox/background/earcons/busy_progress_loop.ogg',
       'chromevox/background/earcons/busy_working_loop.ogg',
       'chromevox/background/earcons/button.ogg',
       'chromevox/background/earcons/check_off.ogg',
       'chromevox/background/earcons/check_on.ogg',
@@ skipping 97 matching lines @@
         {
           'destination': '<(chromevox_dest_dir)/chromevox/background/mathmaps/symbols',
           'files': [
             '<@(chromevox_assets_chromevox_background_mathmaps_symbols)',
           ],
         },
       ],
     },
   ],
 }