Convert DM's -r JSON parsing to SkJSON

Bug: skia:
Change-Id: I987f527dbd742453422073f8e2b28d824f8a62eb
Reviewed-on: https://skia-review.googlesource.com/c/186879
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>