Added Ash.SystemMenu.DefaultView.VisibleRows histogram.

ash/common/system/chromeos/tray_tracing.cc

Index: ash/common/system/chromeos/tray_tracing.cc
diff --git a/ash/common/system/chromeos/tray_tracing.cc b/ash/common/system/chromeos/tray_tracing.cc
index 9c8a7be8363d5c5bb899838277caa906597ed568..d190f964ff1c811140aa7829c2cd6233ef997e4b 100644
--- a/ash/common/system/chromeos/tray_tracing.cc
+++ b/ash/common/system/chromeos/tray_tracing.cc
@@ -66,7 +66,8 @@ class DefaultTracingView : public ActionableView {
 // ash::TrayTracing
 
 TrayTracing::TrayTracing(SystemTray* system_tray)
-    : TrayImageItem(system_tray, IDR_AURA_UBER_TRAY_TRACING), default_(NULL) {
+    : TrayImageItem(system_tray, IDR_AURA_UBER_TRAY_TRACING, UMA_TRACING),
+      default_(nullptr) {
   DCHECK(system_tray);
   WmShell::Get()->system_tray_notifier()->AddTracingObserver(this);
 }