Reviewed by Sam.

        - fix https://bugs.webkit.org/show_bug.cgi?id=19544
          DashboardRegion pieces are leaking

        My recent change introduced a leak of objects owned by each DashboardRegion,
        because I made ~Rect non-virtual. By using a separate RefCounted class for
        each of Rect and DashboardRegion, the leak is gone. It's not clear if the
        DashboardRegion class should really exist -- it doesn't have a DOM binding,
        so it could conceivably be removed some day.

        * css/DashboardRegion.h: Derive from RectBase instead of from Rect.
        * css/Rect.h: Added RectBase. Changed Rect to derive from RectBase.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@34552 268f45cc-cd09-0410-ab3c-d52691b4dbfc