aura: Uber tray should not show up on start screen in kiosk mode.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 64803e8917d50bf4819b8177747c482b8a77d846..b5b6014fec22d66f21ae7c89d9ba4220abfd6ded 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -243,6 +243,8 @@ class DummySystemTrayDelegate : public SystemTrayDelegate {
 
  private:
 
+  virtual bool GetTrayVisibilityOnStartup() OVERRIDE { return true; }
+
   // Overridden from SystemTrayDelegate:
   virtual const std::string GetUserDisplayName() const OVERRIDE {
     return "Über tray Über tray Über tray Über tray";
@@ -632,6 +634,8 @@ void Shell::Init() {
     tray_->AddTrayItem(tray_date);
     tray_->AddTrayItem(tray_accessibility);
     tray_->AddTrayItem(tray_caps_lock);
+
+    tray_->SetVisible(tray_delegate_->GetTrayVisibilityOnStartup());
   }
   if (!status_widget_)
     status_widget_ = internal::CreateStatusArea(tray_.get());