RapidJSON gives us the ability to use JSON in our performance
tests to specific the input. The license is very permissive
and according to the author the performance is excellent.

Chromium's JSON library unfortunately seems quite tied to the
"base" project.

BUG=angle:744

Change-Id: I64e72aad1ed6fa954acd851607032cb48a461348
Reviewed-on: https://chromium-review.googlesource.com/221374Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>