Add a sign-in promo message to the Other Devices menu.

chrome/browser/resources/ntp4/new_tab.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 {
   /* It's necessary to put this here instead of in body in order to get the
      background-size of 100% to work properly */
   height: 100%;
   overflow: hidden;
 }
@@ skipping 405 matching lines @@
 
 /* Put the recently closed menu to the left when it's not visible. */
 #recently-closed-menu-button.invisible {
   -webkit-flex-order: -1;
   visibility: hidden;
 }
 
 #other-sessions-menu-button.invisible {
   display: none;
 }
+
+.other-sessions-promo-message {
+  display: none;
+  padding: 0;
+}
+
+.other-sessions-promo-message:only-child {
+  display: block;
+}
+
+.other-sessions-promo-message p {
+  margin: 0;
+}