Remove static initializer from isolate

We don't have a default isolate anymore, so we can just create the
thread data and keys on demand

BUG=none
R=dcarney@chromium.org, svenpanne@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/337603010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22105 ce2b1a6d-e550-0410-aec6-3dcde31c8c00