Add histogram for how tab closing time. Did some cleanup along the way.

chrome/browser/resources/new_new_tab.js

Index: chrome/browser/resources/new_new_tab.js
===================================================================
--- chrome/browser/resources/new_new_tab.js	(revision 28997)
+++ chrome/browser/resources/new_new_tab.js	(working copy)
@@ -1328,12 +1328,12 @@
 $('list-checkbox').addEventListener('keydown',
                                     getCheckboxHandler(Section.LIST));
 
-window.addEventListener('load', bind(logEvent, global, 'NewTab.Onload', true));
+window.addEventListener('load', bind(logEvent, global, 'Tab.NewTabOnload', true));
 window.addEventListener('load', onDataLoaded);
 
 window.addEventListener('resize', handleWindowResize);
 document.addEventListener('DOMContentLoaded',
-    bind(logEvent, global, 'NewTab.DOMContentLoaded', true));
+    bind(logEvent, global, 'Tab.NewTabDOMContentLoaded', true));
 
 // Whether or not we should send the initial 'GetSyncMessage' to the backend
 // depends on the value of the attribue 'syncispresent' which the backend sets