Don't check for WIN32 define. Use V8_OS_* macros whenever
possible, and if not use _WIN32.

BUG=v8:2300
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/23687003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00