This is for the custom scrollbar painting API on WIndows. This should be removed in favor of a CSS-based solution. Failing that, HostWindow could implement the APIs.
Created attachment 199168 [details]
This doesn't seem to be a problem anymore, although ScrollbarThemeComposite.cpp still includes a lot of headers from WebCore that are currently unused.
Committed r148967: <http://trac.webkit.org/changeset/148967>