Add directive to make TRACE_EVENT macros optional.

Bug: webrtc:11132
Change-Id: I801994ad262e1acff73e4c20afd7a7343b56268c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160654
Commit-Queue: Doudou Kisabaka <doudouk@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29949}

BUILD.gn

@@ -263,6 +263,10 @@ config("common_config") {
     defines += [ "RTC_DISABLE_LOGGING" ]
   }
 
+  if (rtc_disable_trace_events) {
+    defines += [ "RTC_DISABLE_TRACE_EVENTS" ]
+  }
+
   if (build_with_chromium) {
     defines += [
       # NOTICE: Since common_inherited_config is used in public_configs for our