This patch is the first of a multistep process to upgrade JsonCpp
from its current legacy version to the latest version. A minor fix to
how Json::Value is used is included, as well as properly pulling in the
jsoncpp_config where jsoncpp is used, allowing for us to roll Chromium
while Angle uses the deprecated Reader/Writer classes.

After this patch is landed, the Angle dependency in Chromium can be
rolled as part of rolling JsonCpp, then Angle can update to the new
StreamWriter/CharReader classes.

Bug: 961513
Change-Id: Idd1d61270aa0a7e4a91983b4c22a24e2fdaafe49
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1682815Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>