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

ui/views/controls/scrollbar/base_scroll_bar.h

Index: ui/views/controls/scrollbar/base_scroll_bar.h
diff --git a/ui/views/controls/scrollbar/base_scroll_bar.h b/ui/views/controls/scrollbar/base_scroll_bar.h
index c8ede5bcb99e0621fdc64bf6848004fa67b46aa2..fcd38b46c80a39e7fe559b46051ea53899784b98 100644
--- a/ui/views/controls/scrollbar/base_scroll_bar.h
+++ b/ui/views/controls/scrollbar/base_scroll_bar.h
@@ -111,6 +111,10 @@ class VIEWS_EXPORT BaseScrollBar : public ScrollBar,
   virtual void ScrollToPosition(int position);
   virtual int GetScrollIncrement(bool is_page, bool is_positive);
 
+  int contents_size() const { return contents_size_; }
+  int contents_scroll_offset() const { return contents_scroll_offset_; }
+  int viewport_size() const { return viewport_size_; }
+
  private:
   friend class test::ScrollViewTestApi;