Convert //ash from scoped_ptr to std::unique_ptr

ash/system/tray/default_system_tray_delegate.cc

Index: ash/system/tray/default_system_tray_delegate.cc
diff --git a/ash/system/tray/default_system_tray_delegate.cc b/ash/system/tray/default_system_tray_delegate.cc
index 649a9f349a75a9625629f4b0eb360b2b99107c13..2627e926d94c0e0cb57349101f2cba79949201ff 100644
--- a/ash/system/tray/default_system_tray_delegate.cc
+++ b/ash/system/tray/default_system_tray_delegate.cc
@@ -82,7 +82,7 @@ VolumeControlDelegate* DefaultSystemTrayDelegate::GetVolumeControlDelegate()
 }
 
 void DefaultSystemTrayDelegate::SetVolumeControlDelegate(
-    scoped_ptr<VolumeControlDelegate> delegate) {
+    std::unique_ptr<VolumeControlDelegate> delegate) {
   volume_control_delegate_ = std::move(delegate);
 }