rebaseline_server: use new intermediate JSON format as described in https://goto...

rebaseline_server: use new intermediate JSON format as described in https://goto.google.com/ChangingRbsJson

There are still some places in the frontend (HTML+Javascript) code where
we assume we are handling expectations vs actuals, but there are just a few
and we should be able to remove them easily in a coming CL.
At that point, the frontend will work just as well for displaying any set
of image pairs.

BUG=skia:1919
NOTRY=True
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/178253010

git-svn-id: http://skia.googlecode.com/svn/trunk@13598 2bbb7eff-a529-9590-31e7-b0007b416f81