Start removing some pre-md constants and related code.

ash/common/system/tray/tray_details_view.h

Index: ash/common/system/tray/tray_details_view.h
diff --git a/ash/common/system/tray/tray_details_view.h b/ash/common/system/tray/tray_details_view.h
index b7021d6606d00ac9d7e4e935865907883613fd03..ca5902fb4d1c9d78013489c9274b71b0072e1625 100644
--- a/ash/common/system/tray/tray_details_view.h
+++ b/ash/common/system/tray/tray_details_view.h
@@ -12,13 +12,10 @@
 #include "ash/common/system/tray/tray_constants.h"
 #include "ash/common/system/tray/view_click_listener.h"
 #include "base/macros.h"
+#include "base/timer/timer.h"
 #include "ui/views/controls/button/button.h"
 #include "ui/views/view.h"
 
-namespace base {
-class OneShotTimer;
-}  // namespace base
-
 namespace views {
 class BoxLayout;
 class CustomButton;
@@ -137,7 +134,7 @@ class ASH_EXPORT TrayDetailsView : public views::View,
   views::Button* back_button_;
 
   // Used to delay the transition to the default view.
-  std::unique_ptr<base::OneShotTimer> transition_delay_timer_;
+  base::OneShotTimer transition_delay_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayDetailsView);
 };