Removed EnterIsolateIfNeeded and a soon-to-be-useless assertion.

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

Review URL: https://codereview.chromium.org/238773003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20759 ce2b1a6d-e550-0410-aec6-3dcde31c8c00