JsonSerializer: Allow adding values to root node. Previously we would only allow a single group at the root document node. This change allows us to have multiple members. It also changes the API slightly. Instead of calling startDocument, you now can add values and groups on an empty serializer object. It also removes the implicit endGroup call in endDocument. Groups must now be manually ended. Bug: angleproject:5133 Change-Id: Id39016233d73270d1e20d99d59c1729fbcdd8860 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3144213 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com>