This happens on dual-GPU Macs.

Steps to reproduce:

1. Go to https://www.google.com/maps/ and wait for it to load completely.
2. Drag the map around a bit.
3. At some point, dragging freezes for a second.
4. Interact some more, and then stop dragging.
5. Look at the map for 6 seconds.
6. Drag it some more.
7. Encounter another 1 second of jank.

Profile: https://perfht.ml/2ZTAAuW

Expected results:
Ideally, no jank at all. But if there needs to be a GPU switch, I think I'd prefer to have it at the very beginning and then not anymore until I close the tab.

Actual results:
Frequent back-and-forth switching between GPUs.

This is a tricky one. I think maybe there should be a permaban state. GMaps should probably be asking for low-power, honestly.

Google recently announced they would treat "default" power preference as "low-power", so we should just do that, and possibly ditch the (broken!) live-switching implementation for WebGL.

Chrome started shipping this recently, so it's the easiest way to resolve
our dynamic-gpu switching issues for WebGL.

https://hg.mozilla.org/mozilla-central/rev/e12e7c32c8ca

This sounds like a change we might want to let bake for a bit, but LMK if you think this is something we should uplift to Beta for Fx73.