…rs. r=jrmuizel

Few of the counters actually have anything to do with IPC although they all relate to events of layout transactions.

Depends on D69414

Differential Revision: https://phabricator.services.mozilla.com/D69415

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/d2771cdff401747f61d5302cdf7a284eb851b1ce