Merge ScrollbarThemeComposite into ScrollbarTheme.

Previously, every ScrollbarTheme was inheriting from ScrollbarThemeComposite,
so get rid of a level of abstraction.

Also add OVERRIDE to a bunch of places and fix style linter errors.

No behavioral changes.

BUG=None
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/22720002

git-svn-id: svn://svn.chromium.org/blink/trunk@155867 bbb929c8-8fbe-4397-9dbb-9b2b20218538