Use mojo volume interfaces for mash and classic ash.

ash/common/system/tray/system_tray_delegate.cc

Index: ash/common/system/tray/system_tray_delegate.cc
diff --git a/ash/common/system/tray/system_tray_delegate.cc b/ash/common/system/tray/system_tray_delegate.cc
index 4cb07af9f4d9d1401c2ac60f7ac39f5d12d525b8..bcac85614bf65dd2e00dca6d4a189414859a2305 100644
--- a/ash/common/system/tray/system_tray_delegate.cc
+++ b/ash/common/system/tray/system_tray_delegate.cc
@@ -142,13 +142,6 @@ NetworkingConfigDelegate* SystemTrayDelegate::GetNetworkingConfigDelegate()
   return nullptr;
 }
 
-VolumeControlDelegate* SystemTrayDelegate::GetVolumeControlDelegate() const {
-  return nullptr;
-}
-
-void SystemTrayDelegate::SetVolumeControlDelegate(
-    std::unique_ptr<VolumeControlDelegate> delegate) {}
-
 bool SystemTrayDelegate::GetSessionStartTime(
     base::TimeTicks* session_start_time) {
   return false;