This will simplify rolling this out as a Shield experiment.

Originally, we had considered using https://github.com/nhnt11/multipreffer to flip the necessary prefs (performance.adjust_to_machine and performance.low_end_machine) to true.

The problem is, however, that there's code here:

https://searchfox.org/mozilla-central/rev/03ebbdab952409640c6857d835d3040bf6f9e2db/toolkit/components/gfx/SanityTest.js#354

that will clear the performance.low_end_machine pref in later Beta's, which we're starting to come up to.

Either we can aim to only run this experiment in an early Beta (in which case, we'll probably need to wait for Beta 67) with an add-on, or we need to land (and uplift?) a pref that turns this whole thing on or off for the experiment.

Hey Gijs, how safe and simple would it be to write and uplift a patch that allows us to toggle the "low end" state for qualified clients, without wiping it out until the end of the experiment?

Talked to Mike, we're going to make this live behind the single pref we already have, but just make sure it's always off on beta (for now).

https://hg.mozilla.org/mozilla-central/rev/37bfe206487b