Revert "Removed default Isolate."

This reverts commit r21167, cctest/test-serialize has to be fixed first.

TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/267163002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21170 ce2b1a6d-e550-0410-aec6-3dcde31c8c00