Deprecate ChangeLog on bleeding_edge.

- This adds a sentinel to the ChangeLog on bleeding edge.
- Modifying the ChangeLog file directly when pushing to trunk is now no longer possible. If further modifications to the ChangeLog in manual push-to-trunk mode are required (e.g. after the prepare push LGTM), the change log entry temp file must be edited.
- Functionality that is no longer needed is removed.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/199893005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20029 ce2b1a6d-e550-0410-aec6-3dcde31c8c00