Adding instrumentation to locate the source of jankiness.

net/tools/net_watcher/net_watcher.cc

Index: net/tools/net_watcher/net_watcher.cc
diff --git a/net/tools/net_watcher/net_watcher.cc b/net/tools/net_watcher/net_watcher.cc
index b6cf393ee841f51f6eefa4092b7ad98fd61cc601..fb5c8e406d1d4a07b896b750666bd37104cc3fd7 100644
--- a/net/tools/net_watcher/net_watcher.cc
+++ b/net/tools/net_watcher/net_watcher.cc
@@ -14,6 +14,7 @@
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
+#include "base/profiler/scoped_tracker.h"
 #include "base/values.h"
 #include "build/build_config.h"
 #include "net/base/network_change_notifier.h"
@@ -114,6 +115,11 @@ class NetWatcher :
   void OnProxyConfigChanged(
       const net::ProxyConfig& config,
       net::ProxyConfigService::ConfigAvailability availability) override {
+    // TODO(pkasting): Remove ScopedTracker below once crbug.com/455942 is
+    // fixed.
+    tracked_objects::ScopedTracker tracking_profile(
+        FROM_HERE_WITH_EXPLICIT_FUNCTION(
+            "455942 NetWatcher::OnProxyConfigChanged"));
     LOG(INFO) << "OnProxyConfigChanged("
               << ProxyConfigToString(config) << ", "
               << ConfigAvailabilityToString(availability) << ")";