Use button elements instead of div for eject buttons.

ui/file_manager/file_manager/foreground/js/ui/directory_tree.js

Index: ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
index 7c599645d638cb10a8c57b340a7d40cb853fac82..7e84b12fe6ca2287032ead2fcd3818fe5c36a9d3 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/directory_tree.js
@@ -656,7 +656,7 @@ VolumeItem.prototype.setupIcon_ = function(icon, volumeInfo) {
  * @private
  */
 VolumeItem.prototype.setupEjectButton_ = function(rowElement) {
-  var ejectButton = cr.doc.createElement('div');
+  var ejectButton = cr.doc.createElement('button');
   // Block other mouse handlers.
   ejectButton.addEventListener(
       'mouseup', function(event) { event.stopPropagation() });
@@ -664,6 +664,7 @@ VolumeItem.prototype.setupEjectButton_ = function(rowElement) {
       'mousedown', function(event) { event.stopPropagation() });
   ejectButton.className = 'root-eject';
   ejectButton.setAttribute('aria-label', str('UNMOUNT_DEVICE_BUTTON_LABEL'));
+  ejectButton.setAttribute('tabindex', '0');
   ejectButton.addEventListener('click', function(event) {
     event.stopPropagation();
     var unmountCommand = cr.doc.querySelector('command#unmount');