Add a second format to the tracing json/begin_recording endpoint.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 0157f6f0d2c64e8c1d264376d70139f0abdcbe85..757be4296cce44b86cb8a3547f2d0c0f83464a22 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -1334,6 +1334,7 @@ test("content_unittests") {
     "../browser/streams/stream_unittest.cc",
     "../browser/streams/stream_url_request_job_unittest.cc",
     "../browser/tracing/background_tracing_config_unittest.cc",
+    "../browser/tracing/tracing_ui_unittest.cc",
     "../browser/web_contents/aura/overscroll_navigation_overlay_unittest.cc",
     "../browser/web_contents/aura/overscroll_window_animation_unittest.cc",
     "../browser/web_contents/aura/overscroll_window_delegate_unittest.cc",
@@ -1776,6 +1777,7 @@ test("content_unittests") {
     ]
     sources -= [
       "../browser/media/session/audio_focus_manager_unittest.cc",
+      "../browser/tracing/tracing_ui_unittest.cc",
       "../browser/webui/url_data_manager_backend_unittest.cc",
     ]