Add a warning when developing an extension that uses old manifest version.

chrome/browser/resources/extensions/extensions.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.loading * {
   -webkit-transition-duration: 0 !important;
 }
 
 /* Developer mode */
 
@@ skipping 151 matching lines @@
 
 .terminated-reload-link {
   -webkit-margin-end: 2em;
 }
 
 .extension-list-item a {
   -webkit-margin-start: 0.5em;
   display: inline-block;
 }
 
+.install-warnings a {
+  -webkit-margin-start: 0;
+}
+
 .butter-bar,
 .install-warnings,
 .extension-warnings {
   border-radius: 3px;
   margin-top: 5px;
   padding: 2px 5px;
 }
 
 .butter-bar {
   background: rgb(255, 242, 153);
@@ skipping 60 matching lines @@
   opacity: 0;
 }
 
 .extension-list-item-wrapper.may-not-disable .trash {
   visibility: hidden;
 }
 
 .may-not-disable .optional-controls {
   display: none;
 }