Switch DM JSON output to use SkJSONWriter

Bug: skia:
Change-Id: I3ceccd050a294260da54ff8a01a4837eb0cfb369
Reviewed-on: https://skia-review.googlesource.com/c/186866
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>