The goal here is to hook up the ctrl+0 keyboard shortcut to reset the scaling
zoom applied by pinch gestures (on touchscreen or trackpad), in addition to
resetting the reflow zoom (aka full zoom). This patch also makes other mechanisms
to reset the reflow zoom (e.g. clicking on the "100%" label in the hamburger menu)
also reset scaling zoom, which I think makes sense for consistency.
Most of this patch is just plumbing, but I'm unfamiliar with these codepaths
so requesting review from relevant owners to make sure it's sane.
Differential Revision: https://phabricator.services.mozilla.com/D94624
c75bed1584