Enable specification of author email in push-to-trunk.

If not specified, depot tools tend to ask for an email on the command line once in a while, which makes the automated script hang.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/158733002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19223 ce2b1a6d-e550-0410-aec6-3dcde31c8c00