Reland of Improves focus/activation for aura-mus and DesktopNativeWidgetAura

ui/aura/mus/focus_synchronizer_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 UI_AURA_MUS_FOCUS_SYNCHRONIZER_DELEGATE_H_
+#define UI_AURA_MUS_FOCUS_SYNCHRONIZER_DELEGATE_H_
+
+#include <stdint.h>
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+
+class WindowMus;
+
+// Used by FocusSynchronizer to create a change id when focus changes. The
+// change id is then sent to ui::mojom::WindowTree.
+class AURA_EXPORT FocusSynchronizerDelegate {
+ public:
+  virtual uint32_t CreateChangeIdForFocus(WindowMus* window) = 0;
+
+ protected:
+  virtual ~FocusSynchronizerDelegate() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MUS_FOCUS_SYNCHRONIZER_DELEGATE_H_