Make v8 releases script ready for chromium git switch.

This uses chromium commit numbers that are supported after
the chromium git switch. Most functionality is copied from
the bot_update.py script from tools/build.

This also enables mapping bleeding edge revisions to chromium
revisions (previously the data was only retrieved for trunk).

TEST=script_test.py
TEST=tools/push-to-trunk/releases.py -c /path/to/chromium/src --branch recent

TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/497043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23319 ce2b1a6d-e550-0410-aec6-3dcde31c8c00