libAngle: Add Json context serializer library Bug: angleproject:5715 Change-Id: I42319fe30e42d49d7e817b14c211b5ba82a94a42 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2760324 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>