Remove Widget's dependency upon its own inheritor aka ScrollView


This was nasty from an OO design perspective and also complicates
efforts to move ScrollView and Scrollbar to core/page/.

BUG=297477

Review URL: https://codereview.chromium.org/26936002

git-svn-id: svn://svn.chromium.org/blink/trunk@159483 bbb929c8-8fbe-4397-9dbb-9b2b20218538