Make rebaseline-from-try-jobs actually download baselines.

After this CL, `webkit-patch rebaseline-from-try-jobs` should actually
be able to be used for downloading new baselines.

I plan to use http://crrev.com/2106873003 as a sample CL to try it out.

BUG=474273

Review-Url: https://codereview.chromium.org/2118513002
Cr-Commit-Position: refs/heads/master@{#404820}
3 files changed