Use %DebugGetProperty in debug mirror to check for Promise.

R=aandrey@chromium.org, amikhaylova@google.com

Review URL: https://codereview.chromium.org/283373003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21339 ce2b1a6d-e550-0410-aec6-3dcde31c8c00