Have mercy and don't kill the Context::Scope immediately after birth.

BUG=v8::2994
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/68693003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17627 ce2b1a6d-e550-0410-aec6-3dcde31c8c00