I didn't find any usage of PlatformScrollBar.h and PlatformScrollBarWin.cpp.
Created attachment 60971 [details] Patch
Comment on attachment 60971 [details] Patch As best I can tell, you're right that it's not used anywhere and it did pass the EWS bots.
Comment on attachment 60971 [details] Patch Clearing flags on attachment: 60971 Committed r63919: <http://trac.webkit.org/changeset/63919>
All reviewed patches have been landed. Closing bug.