rebaseline_server: allow users to generate SKP diff patches on a shared instance

This will allow developers to maintain SKP baselines without ever running their
own rebaseline_server instance!

For now, the developer must manually apply the resulting patchset to his local
Skia checkout to actually modify expectations; in the near future, we hope to
make the UI upload the patchset to Rietveld on the user's behalf.

BUG=skia:1918
NOTRY=true
R=stephana@google.com, rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/487853004
11 files changed