Disable modifying extensions when in managed mode.

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 235 matching lines @@
   opacity: 0;
 }
 
 .extension-list-item-wrapper.may-not-disable .trash {
   visibility: hidden;
 }
 
 .may-not-disable .optional-controls {
   display: none;
 }
+
+.page:not(.dev-mode) #dev-controls {
+  display: none;
+}
+
+/* Managed mode */
+
+.page:not(.managed-mode) #managed-mode-banner {
+  display: none;
+}
+
+.managed-mode .more-extensions-link {
+  display: none;
+}