Clean up ancestorScrollingLayer

This function should have walked up the containing blocks rather than
the parents. This cl fixes that and removes the useless
ancestorCompositedScrollingLayer.

Note: the layout test here was taken from abarth's patch
https://codereview.chromium.org/340913002/

BUG=None

Review URL: https://codereview.chromium.org/344733003

git-svn-id: svn://svn.chromium.org/blink/trunk@176520 bbb929c8-8fbe-4397-9dbb-9b2b20218538