Make %DebugPushPromise more robust wrt fuzzing.

If %DebugPushPromise and throwing is called outside its intended context,
we may encounter assertion failures.

R=hpayer@chromium.org
BUG=401915
LOG=N

Review URL: https://codereview.chromium.org/453933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23023 ce2b1a6d-e550-0410-aec6-3dcde31c8c00