Clean up updateScrollParent
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@163480 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs