Only GeckoMVMContext really needs the flush, to measure scrolled height
afterwards. Do that explicitly.
This shouldn't change behavior, for the most part; there was a preload
test that relied on the flush when changing DPI to start a run really
clean, but other than that this looks green on try.
Should at best be neutral (just code clean-up), or be a performance
improvement.
In a follow-up, we can possibly remove the DelayedResize code from the
view manager, though I need to think how to possibly coalesce the MVM
reflows, so let's not do that yet.
Differential Revision: https://phabricator.services.mozilla.com/D155385
384b878e04
