…tion. r=nical

Add a first pass at invalidation debugging infrastructure. Also
tidy up the locations that invalidate into a common method, that
sets the invalidation reason.

Differential Revision: https://phabricator.services.mozilla.com/D52128

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/260dfcecdb6287707bd90040a4d2bf7988274ef1