Cache a build asset only when it arrived: a 404 for a chunk asked for while
a deploy was changing over used to be kept as that chunk in that browser.
Refresh the offline copy of the app page after every successful page load,
and when it changes, drop the assets it no longer names along with any
failed response. The same tidy runs when this worker activates, which
clears what earlier workers left. Every deploy's chunks used to stay in
the browser for good.
The cache keeps its name: it also holds what the worker leaves for a tab
to collect.
jcoffey-dev
a607450aaa