Add back a dummy --harmony-promises flag

This is mainly meant to help with the transition of chrome using the
flag to promises being enabled by default

BUG=none
LOG=n
R=rossberg@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/270503005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21188 ce2b1a6d-e550-0410-aec6-3dcde31c8c00