Implement support for chorded braille commands

chrome/common/extensions/api/braille_display_private.idl

 // Copyright 2013 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.
 
 // Braille display access private API.
 namespace brailleDisplayPrivate {
   // Braille display keyboard command.
   enum KeyCommand {
     line_up,
     line_down,
     pan_left,
     pan_right,
     top,
     bottom,
     routing,
     secondary_routing,
     dots,
+    chord,
     standard_key
   };
 
   // A keyboard event.  This is not a standard keyboard event because
   // braille display keyboards look significantly different from standard
   // keyboards.
   dictionary KeyEvent {
     KeyCommand command;
     // 0-based display position for commands that involve a routing key.
     long? displayPosition;
@@ skipping 42 matching lines @@
     static void writeDots(ArrayBuffer cells);
   };
 
   interface Events {
     // Fired when a braille display is connected or disconnected.
     static void onDisplayStateChanged(DisplayState state);
     // Fired when an input event is received from the display.
     static void onKeyEvent(KeyEvent event);
   };
 };