[DevTools] Remove unnecessary scrollbar in settings.

BUG=353961
TBR=vsevik@chromium.org

Review URL: https://codereview.chromium.org/206373002

git-svn-id: svn://svn.chromium.org/blink/trunk@169646 bbb929c8-8fbe-4397-9dbb-9b2b20218538