Implement Sebastien's overlay scrollbars for native UI (Views).

ui/views/controls/scrollbar/base_scroll_bar_thumb.h

 // Copyright (c) 2011 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_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_THUMB_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_THUMB_H_
 
 #include "base/macros.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/views/controls/button/custom_button.h"
@@ skipping 14 matching lines @@
 //
 //  A view that acts as the thumb in the scroll bar track that the user can
 //  drag to scroll the associated contents view within the viewport.
 //
 ///////////////////////////////////////////////////////////////////////////////
 class VIEWS_EXPORT BaseScrollBarThumb : public View {
  public:
   explicit BaseScrollBarThumb(BaseScrollBar* scroll_bar);
   ~BaseScrollBarThumb() override;
 
-  // Sets the size (width or height) of the thumb to the specified value.
-  void SetSize(int size);
+  // Sets the length (width or height) of the thumb to the specified value.
+  void SetLength(int length);
 
   // Retrieves the size (width or height) of the thumb.
   int GetSize() const;
 
   // Sets the position of the thumb on the x or y axis.
   void SetPosition(int position);
 
   // Gets the position of the thumb on the x or y axis.
   int GetPosition() const;
 
   // View overrides:
   gfx::Size GetPreferredSize() const override = 0;
 
  protected:
   // View overrides:
   void OnPaint(gfx::Canvas* canvas) override = 0;
   void OnMouseEntered(const ui::MouseEvent& event) override;
   void OnMouseExited(const ui::MouseEvent& event) override;
   bool OnMousePressed(const ui::MouseEvent& event) override;
   bool OnMouseDragged(const ui::MouseEvent& event) override;
   void OnMouseReleased(const ui::MouseEvent& event) override;
   void OnMouseCaptureLost() override;
 
   CustomButton::ButtonState GetState() const;
   // Update our state and schedule a repaint when the mouse moves over us.
   void SetState(CustomButton::ButtonState state);
+  virtual void OnStateChanged();
+
+  bool IsHorizontal() const;
 
   BaseScrollBar* scroll_bar() { return scroll_bar_; }
 
  private:
   // The BaseScrollBar that owns us.
   BaseScrollBar* scroll_bar_;
 
   int drag_start_position_;
 
   // The position of the mouse on the scroll axis relative to the top of this
   // View when the drag started.
   int mouse_offset_;
 
   // The current state of the thumb button.
   CustomButton::ButtonState state_;
 
   DISALLOW_COPY_AND_ASSIGN(BaseScrollBarThumb);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_BASE_SCROLL_BAR_THUMB_H_