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

ui/views/controls/scrollbar/base_scroll_bar_thumb.h

Index: ui/views/controls/scrollbar/base_scroll_bar_thumb.h
diff --git a/ui/views/controls/scrollbar/base_scroll_bar_thumb.h b/ui/views/controls/scrollbar/base_scroll_bar_thumb.h
index ea16b14ecb4f6efd9b7d9da726fd902bd62572da..7127f434781cdf20f8acbd2c1c081722fc052e41 100644
--- a/ui/views/controls/scrollbar/base_scroll_bar_thumb.h
+++ b/ui/views/controls/scrollbar/base_scroll_bar_thumb.h
@@ -32,8 +32,8 @@ class VIEWS_EXPORT BaseScrollBarThumb : public View {
   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;
@@ -60,6 +60,9 @@ class VIEWS_EXPORT BaseScrollBarThumb : public View {
   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_; }