Looks like it was introduced in http://trac.webkit.org/changeset/97189
The fix is trivial but I'm a little worried about just changing this (since I don't use this platform).
Note that OS(WIN) is never defined so the code in this if isn't used. My patch in 77162 will make code fail to build if it uses OS(WIN).
It is easy to make this mistake (and I did it as well) because there is a PLATFORM(WIN).
Fixed in r106277