ANGLE can use this library to output the test results format used by
the Chromium infrastructure. We have a rapidjson mirror located on
chromium.googlesource.com.

Also updates rapidjson to the latest version and adds GN integration.
Currently this library was unused in ANGLE.

Bug: angleproject:3162
Change-Id: Ie349026f68028e3acc46055655e321c2d710b828
Reviewed-on: https://chromium-review.googlesource.com/c/1487633
Commit-Queue: Jamie Madill <jmadill@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>