Let ChangeLog get auto-generated in push-to-trunk script.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/78683002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17951 ce2b1a6d-e550-0410-aec6-3dcde31c8c00