Adding instrumentation to locate the source of jankiness.

net/proxy/proxy_service.cc

Index: net/proxy/proxy_service.cc
diff --git a/net/proxy/proxy_service.cc b/net/proxy/proxy_service.cc
index a62732d87327c655b03aaf8fcc1307a5c0c5de5d..bd7d8254a1b72566ba84f80af3e5be13886e8b77 100644
--- a/net/proxy/proxy_service.cc
+++ b/net/proxy/proxy_service.cc
@@ -13,6 +13,7 @@
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_loop_proxy.h"
+#include "base/profiler/scoped_tracker.h"
 #include "base/strings/string_util.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/values.h"
@@ -1479,6 +1480,10 @@ scoped_ptr<ProxyService::PacPollPolicy>
 void ProxyService::OnProxyConfigChanged(
     const ProxyConfig& config,
     ProxyConfigService::ConfigAvailability availability) {
+  // TODO(pkasting): Remove ScopedTracker below once crbug.com/455942 is fixed.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+          "455942 ProxyService::OnProxyConfigChanged"));
   // Retrieve the current proxy configuration from the ProxyConfigService.
   // If a configuration is not available yet, we will get called back later
   // by our ProxyConfigService::Observer once it changes.