Fix platform-cygwin.cc to really not use an Isolate

BUG=none
R=jkummerow@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/367953002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22152 ce2b1a6d-e550-0410-aec6-3dcde31c8c00