Fix the tooltip for thumbnail view option in gallery on tabbing

ui/file_manager/gallery/js/gallery.js

 // 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.
 
 /**
  * Overrided metadata worker's path.
  * @type {string}
  */
 ContentMetadataProvider.WORKER_SCRIPT = '/js/metadata_worker.js';
 
@@ skipping 488 matching lines @@
   }
   this.currentMode_ = mode;
   this.currentMode_.addEventListener(
       'sub-mode-change', this.onSubModeChangedBound_);
 
   this.dimmableUIController_.setCurrentMode(
       this.getCurrentMode(), this.getCurrentSubMode());
 
   this.container_.setAttribute('mode', this.currentMode_.getName());
   this.updateSelectionAndState_();
+  this.updateModeButtonAttribute_();
 };
 
 /**
  * Handles sub-mode-change event.
  * @private
  */
 Gallery.prototype.onSubModeChanged_ = function() {
   this.dimmableUIController_.setCurrentMode(
       this.getCurrentMode(), this.getCurrentSubMode());
 };
@@ skipping 12 matching lines @@
  * @private
  */
 Gallery.prototype.onChangeToSlideMode_ = function() {
   if (this.modeSwitchButton_.disabled)
     return;
 
   this.changeCurrentMode_(this.slideMode_);
 };
 
 /**
+ * Update attributes of slide/thumbnail toggle button
+ * @private
+ */
+Gallery.prototype.updateModeButtonAttribute_ = function() {
+  if (this.currentMode_ === this.slideMode_)
+    this.modeSwitchButton_.setAttribute("aria-label", str("GALLERY_THUMBNAIL"));
+  else
+    this.modeSwitchButton_.setAttribute("aria-label", str("GALLERY_SLIDE"));
+};
+
+/**
  * Change current mode.
  * @param {!(SlideMode|ThumbnailMode)} mode Target mode.
  * @param {Event=} opt_event Event that caused this call.
  * @return {!Promise} Resolved when mode has been changed.
  * @private
  */
 Gallery.prototype.changeCurrentMode_ = function(mode, opt_event) {
   return new Promise(function(fulfill, reject) {
     // Do not re-enter while changing the mode.
     if (this.currentMode_ === mode || this.changingMode_) {
@@ skipping 511 matching lines @@
  */
 var initializePromise =
     Promise.all([loadTimeDataPromise, volumeManagerPromise]).
     then(function(args) {
       var volumeManager = args[1];
       gallery = new Gallery(volumeManager);
     });
 
 // Loads entries.
 initializePromise.then(reload);