Adding instrumentation to locate the source of jankiness.

chrome/browser/download/download_resource_throttle.cc

Index: chrome/browser/download/download_resource_throttle.cc
diff --git a/chrome/browser/download/download_resource_throttle.cc b/chrome/browser/download/download_resource_throttle.cc
index 9d9ae92daf0efa8104b9dcb57dc8f80ab277d6bb..d81f0761ead81bf41e45a712c524e33e7fe41205 100644
--- a/chrome/browser/download/download_resource_throttle.cc
+++ b/chrome/browser/download/download_resource_throttle.cc
@@ -5,6 +5,7 @@
 #include "chrome/browser/download/download_resource_throttle.h"
 
 #include "base/bind.h"
+#include "base/profiler/scoped_tracker.h"
 #include "chrome/browser/download/download_stats.h"
 #include "content/public/browser/resource_controller.h"
 
@@ -39,6 +40,11 @@ void DownloadResourceThrottle::WillRedirectRequest(const GURL& new_url,
 }
 
 void DownloadResourceThrottle::WillProcessResponse(bool* defer) {
+  // TODO(vadimt): Remove ScopedTracker below once crbug.com/422516 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "422516 DownloadResourceThrottle::WillProcessResponse"));
+
   WillDownload(defer);
 }