Bug 1088118 - Adding User Timing markers to applications

This requirejs call is run after html_cache_restore has been run, and so window.performance will not be set up correctly in the html_cache_restore.js case where it calls window.performance.

Since this is a temporary shim, I suggest doing what we did for the other performance event code for email:

* copy/paste the code from usertiming.js in html_cache_restore.js, but not the part about the UMD module registration, the global definition on window is enough.
* remove the require for usertiming here.
* remove the index.html script tag change.