Fix the tooltip for thumbnail view option in gallery on tabbing

ui/file_manager/gallery/js/gallery.js

Index: ui/file_manager/gallery/js/gallery.js
diff --git a/ui/file_manager/gallery/js/gallery.js b/ui/file_manager/gallery/js/gallery.js
index 1bbde3214667fb4923938b0c8da90762a0dce4e4..79cae82a7e9e5005d5cdd7197ff033f1f57fe77e 100644
--- a/ui/file_manager/gallery/js/gallery.js
+++ b/ui/file_manager/gallery/js/gallery.js
@@ -506,6 +506,7 @@ Gallery.prototype.setCurrentMode_ = function(mode) {
 
   this.container_.setAttribute('mode', this.currentMode_.getName());
   this.updateSelectionAndState_();
+  this.updateModeButtonAttribute_();
 };
 
 /**
@@ -538,6 +539,17 @@ Gallery.prototype.onChangeToSlideMode_ = function() {
 };
 
 /**
+ * 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.