…r=gw

Second part: trace the updates that are sent to the DataStore, and save
at least the Insert/Remove and ItemUID as part of the wr-capture.
(We could expand this with more info, eg. the actual Keys, later).

TileView then reads them back and generates a color coded report to
overlay with the page view.  This helps to see the types and amounts of
interned primitives that lead to cache invalidations.

Differential Revision: https://phabricator.services.mozilla.com/D60619

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/2e04c320ad2369a369ffba0ab7753a27d37c5ae9