Future-proof redirection to fragments in Gecko

Future-proof redirection to fragments in Gecko

Previously, in Gecko we wouldn't set document.location.hash at all until
the page loaded.  This means that if they fix their bug, so setting
document.location.hash to an id that doesn't yet exist (but will before
load is finished) works properly, we still would only jump to the right
place onload, not ASAP.  Now we'll jump to the right place ASAP, and
then jump again on load, which is better (although still annoying if the
gap ends up being perceptible).

Too bad I can't think of a way to feature-test this.