Cleanup the default Isolate's entry stack.

This adds an ASSERT that the entry_stack_ of the Isolate is
empty upon destruction. The default Isolate is an exception
here, as it may still contain an entry stack item upon
destruction.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/18247004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15435 ce2b1a6d-e550-0410-aec6-3dcde31c8c00