Make scrolling happen relative to the main content frame

Summary: Fixes T7033. When we've reframed the main page content we need to scroll relative to the containing frame, not relative to the window.

Test Plan:
In Safari, Chrome and Firefox, used j/k/J/K keys to navigate diff content.

Tried some other scroll-based beahviors, like jump-to-anchors.

(It looks like the highlighting reticle got slightly derped a while ago, but it's still functional, so I didn't mess with it.)

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7033

Differential Revision: https://secure.phabricator.com/D11490