Have Blink be told of scrollbar changes rather than paying attention itself (Blink side).

BUG=306348
TEST=no visible change; scrollbar changes still work

Review URL: https://codereview.chromium.org/57463002

git-svn-id: svn://svn.chromium.org/blink/trunk@161269 bbb929c8-8fbe-4397-9dbb-9b2b20218538