perf: Add doc on the JSON format.
perf: Add doc on the JSON format.

BUG=skia:
TBR=mtklein

Review URL: https://codereview.chromium.org/1784653002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot