Make rtc_json poisonous in WebRTC

This does not help the attached bugs, but it does allow greater control
over what JSON code is running where. Long-term, the JSON library used
for parsing configuration should likely be a library already present
in Chromium builds, to avoid duplication. And if that happens, then
WebRTC bug 9804 may be passé.

Note that this CL also sorts our poisons alphabetically.

Bug: chromium:895814, webrtc:9804
Change-Id: I70c3efe05a0eba9212895407f73978d8216df920
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158400
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29615}
diff --git a/webrtc.gni b/webrtc.gni
index 6033764..4e0f364 100644
--- a/webrtc.gni
+++ b/webrtc.gni
@@ -366,11 +366,14 @@
   # Encoders and decoders for specific audio codecs such as Opus and iSAC.
   "audio_codecs",
 
-  # Software video codecs (VP8 and VP9 through libvpx).
-  "software_video_codecs",
-
   # Default task queue implementation.
   "default_task_queue",
+
+  # JSON parsing should not be needed in the "slim and modular" WebRTC.
+  "rtc_json",
+
+  # Software video codecs (VP8 and VP9 through libvpx).
+  "software_video_codecs",
 ]
 
 absl_include_config = "//third_party/abseil-cpp:absl_include_config"