Make scroll keys active immediately upon page load
When JX.Scrollbar activates, the page needs to be clicked before scrolling keys work.
Instead, set focus into the content after we set the page frame (if something else isn't already focused).
Also fixes T7042.
Test Plan: In Safari, Chrome and Firefox, scrolling with key commands is now immediately active.
Reviewers: btrahan, chad
Reviewed By: chad
Maniphest Tasks: T7042
Differential Revision: https://secure.phabricator.com/D11508