Add TRACE_EVENT macros to net.

net/disk_cache/blockfile/in_flight_io.cc

Index: net/disk_cache/blockfile/in_flight_io.cc
diff --git a/net/disk_cache/blockfile/in_flight_io.cc b/net/disk_cache/blockfile/in_flight_io.cc
index d5f172b6eddcd6cd66f6da76702e3c8c78d86bb2..f04b8ddae58db19a0aa51e767fc2c86bffe592fc 100644
--- a/net/disk_cache/blockfile/in_flight_io.cc
+++ b/net/disk_cache/blockfile/in_flight_io.cc
@@ -12,6 +12,7 @@
 #include "base/task_runner.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/threading/thread_restrictions.h"
+#include "base/trace_event/trace_event.h"
 
 namespace disk_cache {
 
@@ -24,6 +25,7 @@ void BackgroundIO::OnIOSignalled() {
   // TODO(pkasting): Remove ScopedTracker below once crbug.com/477117 is fixed.
   tracked_objects::ScopedTracker tracking_profile(
       FROM_HERE_WITH_EXPLICIT_FUNCTION("477117 BackgroundIO::OnIOSignalled"));
+  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("net"), "BackgroundIO::OnIOSignalled");
   if (controller_)
     controller_->InvokeCallback(this, false);
 }