Tab value for context menu onclick event should be optional.

chrome/test/data/extensions/platform_apps/context_menu_click/test.js

Index: chrome/test/data/extensions/platform_apps/context_menu_click/test.js
diff --git a/chrome/test/data/extensions/platform_apps/context_menu/test.js b/chrome/test/data/extensions/platform_apps/context_menu_click/test.js
similarity index 70%
copy from chrome/test/data/extensions/platform_apps/context_menu/test.js
copy to chrome/test/data/extensions/platform_apps/context_menu_click/test.js
index 949b201edca2ed19b3398e9d1f8e2abb487f204f..34fa98646a26785b522f41b5c0bc4779d2af597b 100644
--- a/chrome/test/data/extensions/platform_apps/context_menu/test.js
+++ b/chrome/test/data/extensions/platform_apps/context_menu_click/test.js
@@ -11,3 +11,8 @@ chrome.experimental.app.onLaunched.addListener(function() {
         chrome.app.window.create('main.html', {}, function() {});
       });
 });
+
+chrome.contextMenus.onClicked.addListener(function(info, tab) {
+  chrome.test.assertEq(undefined, tab);
+  chrome.test.sendMessage("onClicked fired for " + info.menuItemId);
+});