Whether it's enabled or not must not change while we're running. Let's
not waste time recomputing it. This also makes the VM state tracking
code slightly more lightweight.

Review URL: http://codereview.chromium.org/6825054

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7593 ce2b1a6d-e550-0410-aec6-3dcde31c8c00