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

chrome/browser/ui/overlay/overlay_window.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_OVERLAY_OVERLAY_WINDOW_H_
+#define CHROME_BROWSER_UI_OVERLAY_OVERLAY_WINDOW_H_
+
+#include "ui/base/base_window.h"
+#include "ui/compositor/layer.h"
+
+// This window will always float above other windows. The intention is to show
+// content perpetually while the user is still interacting with the other
+// browser windows.
+class OverlayWindow : public ui::BaseWindow {
+ public:
+  OverlayWindow() {}
+  virtual ~OverlayWindow() {}
+
+  // Returns a created OverlayWindow. This is defined in the platform-specific
+  // implementation for the class.
+  static std::unique_ptr<OverlayWindow> Create();
+
+  virtual void Init() = 0;
+  virtual ui::Layer* GetLayer() = 0;
+
+  // ui::BaseWindow:
+  bool IsActive() const override;
+  bool IsMaximized() const override;
+  bool IsMinimized() const override;
+  bool IsFullscreen() const override;
+  gfx::NativeWindow GetNativeWindow() const override;
+  gfx::Rect GetRestoredBounds() const override;
+  ui::WindowShowState GetRestoredState() const override;
+  gfx::Rect GetBounds() const override;
+  void Show() override;
+  void Hide() override;
+  void ShowInactive() override;
+  void Close() override;
+  void Activate() override;
+  void Deactivate() override;
+  void Maximize() override;
+  void Minimize() override;
+  void Restore() override;
+  void SetBounds(const gfx::Rect& bounds) override;
+  void FlashFrame(bool flash) override;
+  bool IsAlwaysOnTop() const override;
+  void SetAlwaysOnTop(bool always_on_top) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(OverlayWindow);
+};
+
+#endif  // CHROME_BROWSER_UI_OVERLAY_OVERLAY_WINDOW_H_