Make the User Timing API safe to use in MediaWiki

Allow JavaScript code to be annotated via the User Timing API without having to
repeatedly ensure that the API exists by declaring a no-op stub for browsers
that don't implement it. Do it in the startup module so we can use
performance.mark() early. Add an initial performance.mark() call as a use-case.

Task: T105389
Change-Id: Id686073da5baf3fcae36f296e6e50d648a543249