As BREW MP always uses frame flattening, there is no need to render scorllbars. Use the default scrollbar theme with thickness 0.
Created attachment 56563 [details] Patch
Comment on attachment 56563 [details] Patch OK.
Comment on attachment 56563 [details] Patch Clearing flags on attachment: 56563 Committed r59907: <http://trac.webkit.org/changeset/59907>
All reviewed patches have been landed. Closing bug.