Overlay scrollbars expand only when mouse is near thumb

cc/layers/scrollbar_layer_impl_base.h

Index: cc/layers/scrollbar_layer_impl_base.h
diff --git a/cc/layers/scrollbar_layer_impl_base.h b/cc/layers/scrollbar_layer_impl_base.h
index a7a7232d6b0930d549241ed0d7f2c0db8d1353dc..d5bc0f8be412b6af942f84ae011b29c4f0917818 100644
--- a/cc/layers/scrollbar_layer_impl_base.h
+++ b/cc/layers/scrollbar_layer_impl_base.h
@@ -46,7 +46,8 @@ class CC_EXPORT ScrollbarLayerImplBase : public LayerImpl {
   ScrollbarLayerImplBase* ToScrollbarLayer() override;
 
   // Thumb quad rect in layer space.
-  virtual gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeExpandedThumbQuadRect() const;
 
   float thumb_thickness_scale_factor() {
     return thumb_thickness_scale_factor_;
@@ -75,6 +76,9 @@ class CC_EXPORT ScrollbarLayerImplBase : public LayerImpl {
   virtual bool IsThumbResizable() const = 0;
 
  private:
+  gfx::Rect ComputeThumbQuadRectWithThumbThicknessScale(
+      float thumb_thickness_scale_factor) const;
+
   ElementId scroll_element_id_;
   bool is_overlay_scrollbar_;