…e dependencies. r=nical

This reduces the amount of invalidation on some pages very significantly.
It also fixes a number of cases where picture caching "fails", resulting
in everything invalidating every frame.

Differential Revision: https://phabricator.services.mozilla.com/D52275

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/afa3fdcab69155f37dc35daa0cdb22dbb849b907