Add button to add new FSP services to Files app.

ui/file_manager/file_manager/foreground/css/file_manager.css

 /* Copyright (c) 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. */
 
 /* Special attribute used in HTML to hide elements. */
 body[type='folder'] [invisibleif~='folder'],
 body[type='upload-folder'] [invisibleif~='upload-folder'],
 body[type='saveas-file'] [invisibleif~='saveas-file'],
 body[type='open-file'] [invisibleif~='open-file'],
 body[type='open-multi-file'] [invisibleif~='open-multi-file'],
@@ skipping 171 matching lines @@
 #directory-tree .tree-row > .expand-icon {
   flex: none;
   height: 36px;
   left: 3px;
   margin: -12px -2px;
   right: 3px;
   top: 0;
   width: 36px;
 }
 
-#directory-tree .tree-row > .volume-icon {
+#directory-tree .tree-row > .item-icon {
   flex: none;
   height: 16px;
   width: 16px;
 }
 
 #directory-tree .tree-row > .label {
   display: block;
   flex: auto;
   margin: 0 6px;
   overflow-x: hidden;
@@ skipping 1820 matching lines @@
   padding: 20px 15px;
 }
 
 .text-measure {
   pointer-events: none;
   position: absolute;
   top: 0;
   visibility: hidden;
   z-index: -1;
 }