Refators display related functionality into own class

components/mus/ws/display_manager_delegate.h

+// 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.
+
+#ifndef COMPONENTS_MUS_WS_DISPLAY_MANAGER_DELEGATE_H_
+#define COMPONENTS_MUS_WS_DISPLAY_MANAGER_DELEGATE_H_
+
+namespace mus {
+namespace ws {
+
+class Display;
+
+class DisplayManagerDelegate {
+ public:
+  virtual void OnWillDestroyDisplay(Display* display) = 0;
+  virtual void OnFirstDisplayReady() = 0;
+  virtual void OnNoMoreDisplays() = 0;
+
+ protected:
+  virtual ~DisplayManagerDelegate() {}
+};
+
+}  // namespace ws
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_WS_DISPLAY_MANAGER_DELEGATE_H_