This patch updates usages of the JsonCpp library, including the
following changes:

1. Removed unused version variable from the DEPS file.
2. Removed deprecated writer and reader usages, in favor of the new
   builder pattern inside JsonCpp.
3. Modernized usage of the Json::Value types, including some rewrites to
   avoid unnecessary heap allocations, and using the new foreach
   iterators.

Bug: chromium:983223
Change-Id: If26abc8be677d905183a23498fbb81256854525c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2265093Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>