JSONWriter should support adding non containers at max depth

The JSON fuzzing tests failed to write JSON via JSONWriter::Write that
was successfully read with JSONReader.  This was due to an edge case
where we are writing the JSON and have reached max depth (200) but have
non-container leaf entries at that level and nothing deeper.  This case
should be supported by the JSONWriter to match the JSONReader and
JSONParser. Also added a unit test for this case.

Bug: 1138450, 1013934
Change-Id: I98fcd63c957ab4e85a322d923bef18b981220383
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477239
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Chris Davis <chrdavis@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#818212}