Rename OS(UNIX) to OS(POSIX) to be consistent with the rest of chromium and build_config.h.

BUG=286645

Review URL: https://chromiumcodereview.appspot.com/23600014

git-svn-id: svn://svn.chromium.org/blink/trunk@157438 bbb929c8-8fbe-4397-9dbb-9b2b20218538