Initialize scroller to first scrollable element.

On pages where `document.body` is not scrollable, initialise `activatedElement` to the first scrollable element.

This is a big UX win on pages like [this](http://androidxref.com/5.0.0_r2/xref/device/htc/flounder/init.flounder.rc) (which are quite common).  However, might this ever pick some dumb element to scroll?

Fixes #1358.

(Edit: Added `in-post-2.46` label, although it's not; just want to keep track of it.)