Add PlatformScreenDelegate and start implementation.

services/ui/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 SERVICES_UI_WS_DISPLAY_MANAGER_DELEGATE_H_
-#define SERVICES_UI_WS_DISPLAY_MANAGER_DELEGATE_H_
-
-#include "services/ui/public/interfaces/display.mojom.h"
-#include "services/ui/ws/user_id.h"
-
-namespace ui {
-namespace ws {
-
-class Display;
-class WindowManagerState;
-
-class DisplayManagerDelegate {
- public:
-  virtual void OnFirstDisplayReady() = 0;
-  virtual void OnNoMoreDisplays() = 0;
-
-  // Gets the frame decorations for the specified user. Returns true if the
-  // decorations have been set, false otherwise. |values| may be null.
-  virtual bool GetFrameDecorationsForUser(
-      const UserId& user_id,
-      mojom::FrameDecorationValuesPtr* values) = 0;
-
-  virtual WindowManagerState* GetWindowManagerStateForUser(
-      const UserId& user_id) = 0;
-
- protected:
-  virtual ~DisplayManagerDelegate() {}
-};
-
-}  // namespace ws
-}  // namespace ui
-
-#endif  // SERVICES_UI_WS_DISPLAY_MANAGER_DELEGATE_H_