Make scroll keys active immediately upon page load

Summary:
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

Subscribers: epriestley

Maniphest Tasks: T7042

Differential Revision: https://secure.phabricator.com/D11508