…ts in the profiler HUD. r=gw

It's a more useful number since we also report the number of existing and rasterized picture cache tiles.
Also bump the expected number of alpha passes up to 2 since it is rather common.

Differential Revision: https://phabricator.services.mozilla.com/D52134

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/7ce4cf2b6d61a4241708fec5e22f634192a1fe90