Adding jsoncpp in third_party/jsoncpp dir. This is a C++ library to
parse json files.

Bug: angleproject:2785
Change-Id: I12f467ef87c8d4801815c79f211fc57a3a9f4b49
Reviewed-on: https://chromium-review.googlesource.com/1194840Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>