Mac scrollbars are currently painted in ScrollAnimatorMac. This is an odd place for them and entangles painting with the scrolling machinery. All other scrollbars are painted via WebThemeEngine in NativeTheme.

The benefits of doing this, in addition to reducing complexity, is that it'll allow us to unify ScrollAnimatorMac with the other ScrollAnimators. It'll also make it easier to allow for testing of overlay scrollbars which is something that's difficult to do today (and we lack coverage because of it).