[filemanager] Context menu command to pin/unpin file on Drive.

chrome/browser/resources/file_manager/css/file_manager.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. */
 
 html {
   height: 100%;
 }
 
 html.col-resize * {
   cursor: col-resize !important;
@@ skipping 1255 matching lines @@
 #drag-image-container .img-container {
   height: 112px;
   margin: 4px;
   width: 112px;
 }
 
 menu.file-context-menu {
   z-index: 4;
 }
 
+menu.chrome-menu > hr {
+  color: transparent;
+  font-size: 0;
+}
+
 input.common.pin[type='checkbox']:checked::after {
   background-image: -webkit-image-set(
     url('../images/files/ui/pin.png') 1x,
     url('../images/files/ui/2x/pin.png') 2x);
   background-position: 4px 0;
 }
 
 div.offline {
   -webkit-box-pack: center;
   display: -webkit-box;
@@ skipping 298 matching lines @@
 
 #default-action-dialog {
   min-width: 300px;
   width: auto;
 }
 
 .gdrive-welcome-wrapper {
   /* gdrive_welcome.css will override it once loaded. */
   display: none;
 }