Overlay scrollbars expand only when mouse is near thumb

cc/input/single_scrollbar_animation_controller_thinning.h

Index: cc/input/single_scrollbar_animation_controller_thinning.h
diff --git a/cc/input/single_scrollbar_animation_controller_thinning.h b/cc/input/single_scrollbar_animation_controller_thinning.h
index f07806c39f29eca5f3e130aff5aadba99f10bcc4..60e6ef5ca4706df33b16f88c1a05f8ea8ed715b8 100644
--- a/cc/input/single_scrollbar_animation_controller_thinning.h
+++ b/cc/input/single_scrollbar_animation_controller_thinning.h
@@ -33,7 +33,12 @@ class CC_EXPORT SingleScrollbarAnimationControllerThinning {
 
   ~SingleScrollbarAnimationControllerThinning() {}
 
-  bool mouse_is_over_scrollbar() const { return mouse_is_over_scrollbar_; }
+  bool mouse_is_over_scrollbar_thumb() const {
+    return mouse_is_over_scrollbar_thumb_;
+  }
+  bool mouse_is_near_scrollbar_thumb() const {
+    return mouse_is_near_scrollbar_thumb_;
+  }
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
   bool captured() const { return captured_; }
 
@@ -46,7 +51,7 @@ class CC_EXPORT SingleScrollbarAnimationControllerThinning {
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
-  void DidMouseMoveNear(float distance);
+  void DidMouseMoveNear(float distance_to_scrollbar, float distance_to_thumb);
 
  private:
   SingleScrollbarAnimationControllerThinning(
@@ -80,7 +85,8 @@ class CC_EXPORT SingleScrollbarAnimationControllerThinning {
 
   ScrollbarOrientation orientation_;
   bool captured_;
-  bool mouse_is_over_scrollbar_;
+  bool mouse_is_over_scrollbar_thumb_;
+  bool mouse_is_near_scrollbar_thumb_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;