Lower event-tracing log-thread priority.

Lowers risk of the event-tracing thread affecting measurements (and
performance of device). Entirely speculative, but shouldn't hurt.
Timings are still done on the thread that calls the trace macros.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/2013363002 .

Cr-Commit-Position: refs/heads/master@{#12943}

webrtc/base/event_tracer.cc

@@ -160,6 +160,7 @@ class EventLogger final {
     // Finally start, everything should be set up now.
     logging_thread_.Start();
     TRACE_EVENT_INSTANT0("webrtc", "EventLogger::Start");
+    logging_thread_.SetPriority(kLowPriority);
   }
 
   void Stop() {