rebaseline_server: include setA/setB descriptions in output JSON

rebaseline_server: include setA/setB descriptions in output JSON

This is needed for rebaselining using the shared rebaseline_server instance:
by telling the client which git revision was used to fetch expectations, we
allow the client to pass that back along with new baselines (so the server
knows which revision to patch).

This is also handy in that it allows the server to tell the client more about the
different result sets (which files they were generated from).

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

Author: epoger@google.com

Review URL: https://codereview.chromium.org/474813002