Move global V8::IsDead() into the Isolate.

R=yangguo@chromium.org
BUG=v8:2744

Review URL: https://codereview.chromium.org/23549010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16496 ce2b1a6d-e550-0410-aec6-3dcde31c8c00