Make it possible for tests to set up trace event handlers.

webrtc/BUILD.gn

Index: webrtc/BUILD.gn
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index 54a7c1ca56ed79e3c9fd0c619cf5c538e597de26..e69903ac412333a306e7b84a7aff5a29d1f825c7 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -26,6 +26,9 @@ config("common_inherited_config") {
   if (build_with_mozilla) {
     defines += [ "WEBRTC_MOZILLA_BUILD" ]
   }
+  if (rtc_include_tests) {
+    defines += [ "WEBRTC_NON_CONST_TRACE_EVENT_HANDLERS" ]

[kjellander_webrtc, 2017/08/16 12:17:14]

Please add a comment explaining why this is needed and what effect it has.

[kwiberg-webrtc, 2017/08/16 13:11:40]

Please consider always defining this (to 0 or 1, for off/on), and testing it
with #if instead of #ifdef. See
https://groups.google.com/d/msg/discuss-webrtc/tz0Qre1j2QU/iMTSv2lQCAAJ

[ehmaldonado_webrtc, 2017/08/16 13:40:57]

Done.

+  }
   if (build_with_chromium) {
     defines = [
       # TODO(kjellander): Cleanup unused ones and move defines closer to