Fix a platform check to use WEBRTC_WIN instead of OS_WIN.

BUG=4006
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/25169004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7691 4adac7df-926f-26a2-2b94-8c16560cd09d