Intent to unship: window.mozPaintCount.

[Emilio Cobos Álvarez]

In bug 1591968 I'm trying to add a default-off preference for
window.mozPaintCount.

This Gecko-only API returns the amount of times that a page has been
painted to the window (sorta, it's a bit unclear what it is exactly
counting, given the point of the code where the count is incremented[1],
at least to me, or how would it interact with Fission).

I don't think this API should be exposed to content as it exposes stuff
that we generally don't want to expose (and makes some information that
a page would otherwise need to obtain via side-channels readily available).

The preference will be dom.mozPaintCount.enabled. I don't really expect
anyone to be using this with any legit purpose (there were only internal
tests, and a quick search only yields various types of fps-counters that
deal with the lack of the property). But let me know if you want me to
add a use-counter and such for a few releases before doing this,
probably would be better.

Thanks,

-- Emilio

[1]:
https://searchfox.org/mozilla-central/rev/74cc0f4dce444fe0757e2a6b8307d19e4d0e0212/layout/base/PresShell.cpp#6055