Make the repeat button accessible by keyboard.

ui/file_manager/audio_player/elements/repeat_button.js

 // 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.
 
 /**
  * Repeat Button.
  *
  * This is for repeat button in Control Panel for Audio Player.
  */
 Polymer({
   is: 'repeat-button',
 
+  hostAttributes: {
+    role: 'button',
+    tabindex: 0
+  },
+
+  behaviors: [
+    Polymer.IronButtonState,
+    Polymer.IronControlState
+  ],
+
   properties: {
     'repeatMode': {
       type: String,
       notify: true,
       reflectToAttribute: true
     }
   },
 
   listeners: {
     tap: '_tapHandler'
   },
 
+  observers: [
+    '_focusedChanged(receivedFocusFromKeyboard)'
+  ],
+
+  _focusedChanged: function(receivedFocusFromKeyboard) {
+    if (receivedFocusFromKeyboard) {
+      this.classList.add('keyboard-focus');
+    } else {
+      this.classList.remove('keyboard-focus');
+    }
+  },
+
   /**
    * Initialize member variables.
    */
   created: function() {
     /**
      * @private {Array<string>}
      */
     this.modeName_ = [
       "no-repeat",
       "repeat-all",
@@ skipping 21 matching lines @@
 
   /**
    * Whether or not the button is active, which means it should be toggled.
    * @param {string} mode Current mode name
    * @return {boolean} True if the mode is repeat.
    */
   isActive: function(mode) {
     return mode === "repeat-all" || mode === "repeat-one";
   },
 });