Add keyboard overlay help for the Search key as a Function key extended keyboard shortcuts.

chrome/browser/resources/chromeos/keyboard_overlay.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.
  */
 
 body {
   -webkit-user-select: none;
   background: #fff;
   margin: 0;
   overflow: hidden;
@@ skipping 71 matching lines @@
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-ctrl {
   border-color: rgb(93, 128, 199);
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-alt {
   border-color: rgb(184, 84, 84);
 }
 
+.keyboard-overlay-key.is-shortcut.modifier-search {
+  border-color: rgb(204, 204, 80);
+}
+
 .keyboard-overlay-key.is-shortcut.modifier-shift.modifier-ctrl {
   border-color: rgb(121, 172, 143);
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-shift.modifier-alt {
   border-color: rgb(191, 189, 121);
 }
 
 .keyboard-overlay-key.is-shortcut.modifier-ctrl.modifier-alt {
   border-color: rgb(158, 84, 206);
@@ skipping 52 matching lines @@
 
 .keyboard-overlay-key.pressed.is-alt.modifier-ctrl {
   background: -webkit-linear-gradient(rgb(118, 54, 143), rgb(82, 33, 40));
 }
 
 .keyboard-overlay-key.pressed.is-alt.modifier-shift.modifier-ctrl {
   background: -webkit-linear-gradient(top left, rgb(115, 95, 41),
                                                 rgb(80, 36, 27));
 }
 
+.keyboard-overlay-key.pressed.is-search {
+  background: -webkit-linear-gradient(rgb(242, 242, 126), rgb(186, 186, 52));
+}
+
 .keyboard-overlay-shortcut-text {
   -webkit-box-flex: 1;
   -webkit-box-ordinal-group: 1;
   color: #fff;
   text-align: center;
 }
 
 .keyboard-overlay-key-text {
   -webkit-box-ordinal-group: 2;
   padding-bottom: 1px;
   text-align: center;
 }