Reduce usage of webkit prefixes in extension examples

chrome/common/extensions/docs/examples/api/downloads/download_manager/popup.css

 /* Copyright (c) 2013 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. */
 
 #outer,
 #empty,
 #open-folder,
 .file-url,
 .time-left,
 .more-left {
@@ skipping 185 matching lines @@
     color-stop(.25, rgba(255, 255, 255, .2)),
     color-stop(.25, transparent),
     color-stop(.5, transparent),
     color-stop(.5, rgba(255, 255, 255, .2)),
     color-stop(.75, rgba(255, 255, 255, .2)),
     color-stop(.75, transparent),
     to(transparent));
   z-index: 1;
   -webkit-background-size: 50px 50px;
   background-size: 50px 50px;
-  -webkit-animation: move 2s linear infinite;
+  animation: move 2s linear infinite;
   overflow: hidden;
 }
 
-@-webkit-keyframes move {
+@keyframes move {
   0% { background-position: 0 0; }
   100% { background-position: 50px 50px; }
 }
 
 .url {
   color: grey;
   max-width: 700px;
   overflow: hidden;
   display: block;
 }
 
 #text-width-probe {
   position: absolute;
   top: -100px;
   left: 0;
 }