mash: Move SystemTrayDelegate ownership to WmShell

ash/common/wm_shell.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/wm_shell.h"
 
+#include "ash/common/system/tray/system_tray_delegate.h"
+
 namespace ash {
 
 // static
 WmShell* WmShell::instance_ = nullptr;
 
 // static
 void WmShell::Set(WmShell* instance) {
   instance_ = instance;
 }
 
 // static
 WmShell* WmShell::Get() {
   return instance_;
 }
 
+WmShell::WmShell() {}
+
+WmShell::~WmShell() {}
+
+void WmShell::SetSystemTrayDelegate(
+    std::unique_ptr<SystemTrayDelegate> delegate) {
+  // TODO(jamescook): Create via ShellDelegate once it moves to //ash/common.
+  system_tray_delegate_ = std::move(delegate);
+}
+
 }  // namespace ash