Call window.onscroll event at the end of scrolling.

Remove some old cruft code that isn't necessary anymore.  We almost always send
the scroll event to webkit except when restoring the scroll position when going
back.

I have verified that the scroll position is restored when going between pages
and that gmail receives the final scroll position.

Bug: 3187015
Change-Id: I0b1dfd1096d44247729bdd13bdad908310c2f19a