scroll the selection when dragging near the edge

Also move a debug statement so it can report the
extra selection choice.

This does not scrolls overflow divs for now; once
Pat has that code checked in, this can adapt to
work for that as well.

requires a companion change in external/webkit

bug:3186988
Change-Id: I071ecd31d589e9ef715d2ecb12b1768696075686