Adding instrumentation to locate the source of jankiness.

net/url_request/url_request_job.cc

Index: net/url_request/url_request_job.cc
diff --git a/net/url_request/url_request_job.cc b/net/url_request/url_request_job.cc
index 0fe4c7c71fb2fc4cf220e29d2a8d2f5316f42915..09e462b343f78864a119c52ae4db85aad2c0513c 100644
--- a/net/url_request/url_request_job.cc
+++ b/net/url_request/url_request_job.cc
@@ -8,6 +8,7 @@
 #include "base/compiler_specific.h"
 #include "base/message_loop/message_loop.h"
 #include "base/power_monitor/power_monitor.h"
+#include "base/profiler/scoped_profile.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "net/base/auth.h"
@@ -299,6 +300,11 @@ void URLRequestJob::NotifyBeforeNetworkStart(bool* defer) {
 }
 
 void URLRequestJob::NotifyHeadersComplete() {
+  // TODO(vadimt): Remove ScopedProfile below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedProfile tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLRequestJob::NotifyHeadersComplete"));
+
   if (!request_ || !request_->has_delegate())
     return;  // The request was destroyed, so there is no more work to do.