[OverlayWindow] Add platform-independent window and views implementation.

chrome/browser/ui/views/overlay/overlay_window_views.h

+// Copyright 2017 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 CHROME_BROWSER_UI_VIEWS_OVERLAY_OVERLAY_WINDOW_VIEWS_H_
+#define CHROME_BROWSER_UI_VIEWS_OVERLAY_OVERLAY_WINDOW_VIEWS_H_
+
+#include "chrome/browser/ui/overlay/overlay_window.h"
+
+namespace views {
+class Widget;
+}
+
+// The Views implementation of OverlayWindow.
+class OverlayWindowViews : public OverlayWindow {
+ public:
+  OverlayWindowViews();
+  ~OverlayWindowViews() override;
+
+  // OverlayWindow:
+  void Init() override;
+  bool IsActive() const override;
+  void Show() override;
+  void Hide() override;
+  void Close() override;
+  void Activate() override;
+  bool IsAlwaysOnTop() const override;
+  ui::Layer* GetLayer() override;
+  gfx::NativeWindow GetNativeWindow() const override;
+  gfx::Rect GetBounds() const override;
+
+ private:
+  std::unique_ptr<views::Widget> widget_;
+
+  DISALLOW_COPY_AND_ASSIGN(OverlayWindowViews);
+};
+
+#endif  // CHROME_BROWSER_UI_VIEWS_OVERLAY_OVERLAY_WINDOW_VIEWS_H_