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}

webrtc.gni

@@ -245,6 +245,9 @@ declare_args() {
 
   # Set this to true to fully remove logging from WebRTC.
   rtc_disable_logging = false
+
+  # Set this to true to disable trace events.
+  rtc_disable_trace_events = false
 }
 
 # Make it possible to provide custom locations for some libraries (move these