Convert //ash from scoped_ptr to std::unique_ptr

ash/system/tray/system_tray.cc

Index: ash/system/tray/system_tray.cc
diff --git a/ash/system/tray/system_tray.cc b/ash/system/tray/system_tray.cc
index 9fc1b8aa5566b14f1707249e368ec45cc8b28668..211570870c7057dd021b1e584d8692b7a91c53c7 100644
--- a/ash/system/tray/system_tray.cc
+++ b/ash/system/tray/system_tray.cc
@@ -117,8 +117,8 @@ class SystemBubbleWrapper {
   bool is_persistent() const { return is_persistent_; }
 
  private:
-  scoped_ptr<SystemTrayBubble> bubble_;
-  scoped_ptr<TrayBubbleWrapper> bubble_wrapper_;
+  std::unique_ptr<SystemTrayBubble> bubble_;
+  std::unique_ptr<TrayBubbleWrapper> bubble_wrapper_;
   bool is_persistent_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemBubbleWrapper);