Move rtc_json code from API dir, enable unit test, unmark testonly

This change does three things:
 - Move rtc_json into rtc_base/strings/, a non-API directory more fitting to
   its purpose.
 - Make a target for the currently unused json_unittest.
 - Make the code available for use in non-test code again.

Bug: webrtc:9802
Change-Id: Id964a8a4b47b732a962a364894a4dbd3e7f4650f
Reviewed-on: https://webrtc-review.googlesource.com/103126
Commit-Queue: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24932}
diff --git a/examples/peerconnection/client/conductor.cc b/examples/peerconnection/client/conductor.cc
index 65d4aca..c25259c 100644
--- a/examples/peerconnection/client/conductor.cc
+++ b/examples/peerconnection/client/conductor.cc
@@ -24,8 +24,8 @@
 #include "modules/audio_processing/include/audio_processing.h"
 #include "modules/video_capture/video_capture_factory.h"
 #include "rtc_base/checks.h"
-#include "rtc_base/json.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/strings/json.h"
 
 // Names used for a IceCandidate JSON object.
 const char kCandidateSdpMidName[] = "sdpMid";