Swap addOnloadHook to something that actually works.

The ext.codereview.loaddiff module isn't loaded at the top and hence this
code would throw out errors (at least on Internet Explorer 8, but likely
on other browsers, too). By loading it in JS via ResourceLoader it works
as expected.

Change-Id: Ia8fa5c73891d8436344c00c50f96fdbcc72242af