resourceloader: Optimise mw.now() definition

resourceloader: Optimise mw.now() definition

In a microbenchmark this wouldn't register given it's just property
access, but the 'timing' and 'navigationStart' properties are
non-trivial getter accessors on their first call, and that's worth
deferring to when it is needed, instead of blocking the definition
of mw.loader early on.

Also remove the redundant wrapper around Date.now(), which is a
static and otherwise detachable function, it does not need to
be bound or wrapped. jQuery defines its shortcut the same way,
as `jQuery.now = Date.now`.

Change-Id: Id621b08fbcce886318bae76ea4c47d50fc9d88e9