Adding instrumentation to locate the source of jankiness.

net/url_request/url_fetcher_core.cc

Index: net/url_request/url_fetcher_core.cc
diff --git a/net/url_request/url_fetcher_core.cc b/net/url_request/url_fetcher_core.cc
index 5ea39219363ab4652cedf04166a65911d17811a3..51fa40ae0b246440b52f4b94b6b9a3b88f10e94f 100644
--- a/net/url_request/url_fetcher_core.cc
+++ b/net/url_request/url_fetcher_core.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
+#include "base/profiler/scoped_tracker.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
@@ -390,6 +391,11 @@ void URLFetcherCore::OnReceivedRedirect(URLRequest* request,
 }
 
 void URLFetcherCore::OnResponseStarted(URLRequest* request) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLFetcherCore::OnResponseStarted"));
+
   DCHECK_EQ(request, request_.get());
   DCHECK(network_task_runner_->BelongsToCurrentThread());
   if (request_->status().is_success()) {
@@ -418,6 +424,11 @@ void URLFetcherCore::OnCertificateRequested(
 
 void URLFetcherCore::OnReadCompleted(URLRequest* request,
                                      int bytes_read) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLFetcherCore::OnReadCompleted"));
+
   DCHECK(request == request_);
   DCHECK(network_task_runner_->BelongsToCurrentThread());