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/BUILD.gn b/BUILD.gn
index b1fc7b4..85b0769 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -465,6 +465,7 @@
       "rtc_base:rtc_base_tests_main",
       "rtc_base:rtc_base_tests_utils",
       "rtc_base:rtc_base_unittests",
+      "rtc_base:rtc_json_unittests",
       "rtc_base:rtc_numerics_unittests",
       "rtc_base:rtc_task_queue_unittests",
       "rtc_base:sequenced_task_checker_unittests",