Adding instrumentation to locate the source of jankiness.

net/url_request/url_fetcher_response_writer.cc

Index: net/url_request/url_fetcher_response_writer.cc
diff --git a/net/url_request/url_fetcher_response_writer.cc b/net/url_request/url_fetcher_response_writer.cc
index fcc8cbf38ffc4129245cd2cf2ceb63466e112a67..3c66dd34f3945a81c52e2965a1b7b1c1501581f3 100644
--- a/net/url_request/url_fetcher_response_writer.cc
+++ b/net/url_request/url_fetcher_response_writer.cc
@@ -6,6 +6,7 @@
 
 #include "base/files/file_util.h"
 #include "base/location.h"
+#include "base/profiler/scoped_profile.h"
 #include "base/sequenced_task_runner.h"
 #include "base/task_runner_util.h"
 #include "net/base/file_stream.h"
@@ -173,6 +174,11 @@ void URLFetcherFileWriter::DidCreateTempFile(const CompletionCallback& callback,
 
 void URLFetcherFileWriter::DidOpenFile(const CompletionCallback& callback,
                                        int result) {
+  // TODO(vadimt): Remove ScopedProfile below once crbug.com/423948 is fixed.
+  tracked_objects::ScopedProfile tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "423948 URLFetcherFileWriter::DidOpenFile"));
+
   if (result == OK)
     owns_file_ = true;
   else