Bug 908724 - Avoid #including nsPresContext.h in nsIScrollableFrame.h; r=mats

nsPresContext.h pulls in a large number of headers which should not
be needed in nsIScrollableFrame, if we just move ScrollbarStyles to
its own header
Bug 905370 - Move SelfHostedClass list to JSRuntime. r=bhackett,shu

--HG--
extra : rebase_source : e45e182038fdc5267aa9b50853eefb43c7f9cf54