Overlay scrollbars expand only when mouse is near thumb

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index c34273e57ee835c1cd082014b65aae4544008ae6..96055dba0568fbe7414de705bafd7a39ba1d1971 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -82,13 +82,15 @@ class CC_EXPORT ScrollbarAnimationController {
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
-  void DidMouseMoveNear(ScrollbarOrientation, float);
+  void DidMouseMoveNear(ScrollbarOrientation orientation,
+                        const gfx::PointF& device_viewport_point);
 
   // Called when Blink wants to show the scrollbars (via
   // ScrollableArea::showOverlayScrollbars).
   void DidRequestShowFromMainThread();
 
-  bool MouseIsOverScrollbar(ScrollbarOrientation orientation) const;
+  bool MouseIsOverScrollbarThumb(ScrollbarOrientation orientation) const;
+  bool MouseIsNearScrollbarThumb(ScrollbarOrientation orientation) const;
   bool MouseIsNearScrollbar(ScrollbarOrientation orientation) const;
   bool MouseIsNearAnyScrollbar() const;
 
@@ -112,6 +114,7 @@ class CC_EXPORT ScrollbarAnimationController {
                                base::TimeDelta thinning_duration);
 
   ScrollbarSet Scrollbars() const;
+  ScrollbarLayerImplBase* GetScrollbar(ScrollbarOrientation orientation) const;
   SingleScrollbarAnimationControllerThinning& GetScrollbarAnimationController(
       ScrollbarOrientation) const;