I found three places were jsoncpp is used in sources, all of them are not built as part of qtwebengine: - src/3rdparty/chromium/third_party/skia : used for benchmarks only; - src/3rdparty/chromium/third_party/libjingle: used when built for Android; - src/3rdparty/chromium/third_party/webrtc : used for standalone WebRTC builds.
Thanks, fixed in git. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=90f45d0172ad22d6b8a6e77025499156b704dcc9