Don't call into ScrollView from Scrollbar.

This will be a layering violation when ScrollView code moves into Source/core.

BUG=417774

Review URL: https://codereview.chromium.org/621653002

git-svn-id: svn://svn.chromium.org/blink/trunk@183316 bbb929c8-8fbe-4397-9dbb-9b2b20218538