This points to a mirror of the Chromium jsoncpp folder. The
Chromium version includes a BUILD.gn that we can use.

Bug: angleproject:2794
Change-Id: I08f34e693d547cd01caefdeddaf43ce4b4bbc29d
Reviewed-on: https://chromium-review.googlesource.com/1208510Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>