Make SKP bench JSON ouput better

BUG=skia:
NOTREECHECKS=true
R=bensong@google.com, jcgregorio@google.com, bsalomon@google.com, robertphillips@google.com, rmistry@google.com

Author: kelvinly@google.com

Review URL: https://codereview.chromium.org/329993008
6 files changed