Make auto-roll testable.

Refactor the mock code for easier reuse. Mock out web requests.

TEST=python -m unittest test_scripts
BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/77453009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17987 ce2b1a6d-e550-0410-aec6-3dcde31c8c00