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

ui/views/controls/scroll_view.h

Index: ui/views/controls/scroll_view.h
diff --git a/ui/views/controls/scroll_view.h b/ui/views/controls/scroll_view.h
index 8e30b169edf4447aca7affbbb453c2babefd2233..288be675d7ac34a7fd4d7e53af00c3dc83c9733d 100644
--- a/ui/views/controls/scroll_view.h
+++ b/ui/views/controls/scroll_view.h
@@ -97,8 +97,6 @@ class VIEWS_EXPORT ScrollView : public View, public ScrollBarController {
   void Layout() override;
   bool OnKeyPressed(const ui::KeyEvent& event) override;
   bool OnMouseWheel(const ui::MouseWheelEvent& e) override;
-  void OnMouseEntered(const ui::MouseEvent& event) override;
-  void OnMouseExited(const ui::MouseEvent& event) override;
   void OnScrollEvent(ui::ScrollEvent* event) override;
   void OnGestureEvent(ui::GestureEvent* event) override;
   const char* GetClassName() const override;