Add ChromeVox panel and implement caption display functionality.

chrome/browser/resources/chromeos/chromevox/cvox2/background/panel_command.js

+// 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.
+
+/**
+ * @fileoverview Commands to pass from the ChromeVox background page context
+ * to the ChromeVox Panel.
+ */
+
+goog.provide('PanelCommand');
+goog.provide('PanelCommandType');
+
+/**
+ * Create one command to pass to the ChromeVox Panel.
+ * @param {PanelCommandType} type The type of command.
+ * @param {string|ArrayBuffer=} opt_data
+ *     Optional data associated with the command.
+ * @constructor
+ */
+PanelCommand = function(type, opt_data) {
+  this.type = type;
+  this.data = opt_data;
+};
+
+/**
+ * Send this command to the ChromeVox Panel window.
+ */
+PanelCommand.prototype.send = function() {
+  var views = chrome.extension.getViews();
+  for (var i = 0; i < views.length; i++) {
+    if (views[i].location.href.indexOf('background/panel.html') > 0) {
+      views[i].postMessage(JSON.stringify(this), window.location.origin);
+    }
+  }
+};
+
+/**
+ * Possible panel commands.
+ * @enum {string}
+ */
+PanelCommandType = {
+  CLEAR_SPEECH: 'clear_speech',
+  ADD_NORMAL_SPEECH: 'add_normal_speech',
+  ADD_ANNOTATION_SPEECH: 'add_annotation_speech',
+  UPDATE_BRAILLE_TEXT: 'update_braille_text',
+  UPDATE_BRAILLE_CELLS: 'update_braille_cells'

[Peter Lundblad, 2015/11/06 15:00:58]

Since cells and text should always be updated together, I think it would be
clearer to have it as one command. Just make the command contain a subobject
instead of a string|ArrayBuffer.
This also makes sense if we ever want to support aligning the characters
and cells similar to what we do in the corresponding feature on BrailleBack
on Android. I don't know how much effort that is worth, though.

[dmazzoni, 2015/11/06 21:01:01]

Done.

I already adjusted the CSS so that they line up as well as possible without
doing something more advanced line one character at a time.

+};