Add launcherPage.hide() API.

chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js

 // Copyright 2014 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.
 
 chrome.launcherPage.onTransitionChanged.addListener(function(progress) {
   if (progress == 0) {
     chrome.test.sendMessage('onPageProgressAt0');
   } else if (progress == 1) {
     // Push 2 launcher page subpages.
     chrome.launcherPage.pushSubpage(function() {
@@ skipping 13 matching lines @@
       chrome.test.sendMessage('launcherPageDisabled');
   });
 }
 
 function enableCustomLauncherPage() {
   chrome.launcherPage.setEnabled(true, function() {
       chrome.test.sendMessage('launcherPageEnabled');
   });
 }
 
+function hideCustomLauncherPage() {
+  chrome.launcherPage.hide(function() {
+      chrome.test.sendMessage('launcherPageHidden');
+  });
+}
+
 document.addEventListener('DOMContentLoaded', function() {
   chrome.test.sendMessage('Launched');
 });
 
 var textfield = document.getElementById('textfield');
 textfield.onfocus = function() {
   chrome.test.sendMessage('textfieldFocused');
 };
 
 textfield.onblur = function() {
   chrome.test.sendMessage('textfieldBlurred');
 };