Clean up updateScrollParent

Current code is clumsy and brittle.

R=shawnsingh@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/107183003

git-svn-id: svn://svn.chromium.org/blink/trunk@163480 bbb929c8-8fbe-4397-9dbb-9b2b20218538