Garden-o-matic: Fix rebaseline-json results handling

The rebaseline-json results are returned as a JSON object
with |return_code| and optional error |output| fields.

R=eseidel,dglazkov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/183383008

git-svn-id: svn://svn.chromium.org/blink/trunk@168446 bbb929c8-8fbe-4397-9dbb-9b2b20218538