Removed default Isolate.

There is probably room for more cleanup after this...

BUG=359977
LOG=y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/262163006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21167 ce2b1a6d-e550-0410-aec6-3dcde31c8c00