Move the JSON parser into a general Parser interface

This also moves json_parser into a parsers subdir. More parsers that
run in the same sandbox will be added in followups.

R=csharp

Bug: 830892
Change-Id: I8c73d7a88d03f62dd817bd9c834bd9a61783eb5b
Reviewed-on: https://chromium-review.googlesource.com/c/1329893
Commit-Queue: Joe Mason <joenotcharles@google.com>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608084}
46 files changed