[FileBrowser] Migrated ComboButton to menu as dropdown.

chrome/browser/resources/file_manager/css/common.css

 /* Copyright (c) 2012 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. */
 
 /* This file contains "borrowed" copy of standard styles. To simplify merging,
  * when altering, please preserve original property value by adding comments.
  */
 input.common[type='checkbox'],
 input.common[type='radio'] {
   -webkit-appearance: none;
@@ skipping 183 matching lines @@
 }
 
 /* "chrome-menu" class overrides some standard menu.css styles, to make custom
    menus in FileBrowser look like native ChromeOS menus. */
 
 menu.chrome-menu {
   border: 1px solid rgba(0, 0, 0, 0.2);
   border-radius: 2px;
   outline: none;
   padding: 2px 0;
+  z-index: 15;
 }
-
 menu.chrome-menu > :not(hr) {
+  background-position: 6px center;
+  background-repeat: no-repeat;
   color: rgb(34, 34, 34);
   height: 30px;
   line-height: 28px;
   padding-left: 28px;
   padding-right: 20px;
 }
 
 menu.chrome-menu > :not(hr)[disabled] {
   background-color: rgb(255, 255, 255);
   color: rgb(153, 153, 153);
 }
 
 menu.chrome-menu > :not(hr):not([disabled])[selected],
 menu.chrome-menu > :not(hr):not([disabled])[selected]:active {
   background-color: rgb(241, 241, 241);
   color: rgb(34, 34, 34);
 }
 
 menu.chrome-menu > hr {
   background: rgb(218, 218, 218);
   height: 1px;
   margin: 2px 0;
 }
 
 menu.chrome-menu > :not(hr)[checked] {
-  background: url('../images/common/check_no_box.png') no-repeat 2px center;
+  background-image: url('../images/common/check_no_box.png');
+  background-position: 2px center;
 }
 
 menu.chrome-menu > [checked]:before {
   display: none;
 }