[FileBrowser] Added DefaultAction dialog to choose default action.

chrome/browser/resources/file_manager/js/combobutton.js

Index: chrome/browser/resources/file_manager/js/combobutton.js
diff --git a/chrome/browser/resources/file_manager/js/combobutton.js b/chrome/browser/resources/file_manager/js/combobutton.js
index 03206e55c08e3ab4539bf281940ea03da0fe3cb4..fdb98f672802cfd7e0cf78d0b6a9b4c65354a300 100644
--- a/chrome/browser/resources/file_manager/js/combobutton.js
+++ b/chrome/browser/resources/file_manager/js/combobutton.js
@@ -29,12 +29,23 @@ cr.define('cr.ui', function() {
       this.multiple = false;
     },
 
+    /**
+     * Adds item to drop-down list.
+     * @param {Object} item Item to add. Should have label and iconURL fields.
+     */
     addItem: function(item) {
       this.multiple = true;
       this.menu.addMenuItem(item).data = item;
     },
 
     /**
+     * Adds separator to drop-down list.
+     */
+    addSeparator: function() {
+      this.menu.addSeparator();
+    },
+
+    /**
      * Default item to fire on combobox click
      */
     get defaultItem() {