Move preferred scrollbar style preference change listening from renderer to browser, 3 of 3.

This removes the preferred style listener from Blink, now that the browser
sends the value when it changes over IPC.

BUG=306348

Review URL: https://codereview.chromium.org/136333009

git-svn-id: svn://svn.chromium.org/blink/trunk@165714 bbb929c8-8fbe-4397-9dbb-9b2b20218538