- Call push-to-trunk through python not through the shell
- Restore tree state on script errors
- Mock out python call in unit tests
- The actual call stays behind a flag

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/141633003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18664 ce2b1a6d-e550-0410-aec6-3dcde31c8c00