Add launcherPage.hide() API.

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

Index: chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js
diff --git a/chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js b/chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js
index ce4fdc07ac7132465ed16042a902cded91dc09a0..4ccc500337d0dd4ca9d737808357ffbcceb40195 100644
--- a/chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js
+++ b/chrome/test/data/extensions/platform_apps/custom_launcher_page/main.js
@@ -31,6 +31,12 @@ function enableCustomLauncherPage() {
   });
 }
 
+function hideCustomLauncherPage() {
+  chrome.launcherPage.hide(function() {
+      chrome.test.sendMessage('launcherPageHidden');
+  });
+}
+
 document.addEventListener('DOMContentLoaded', function() {
   chrome.test.sendMessage('Launched');
 });