Keep expanded if mouse moves off of scrollbar while dragging

cc/input/scrollbar_animation_controller_thinning.h

Index: cc/input/scrollbar_animation_controller_thinning.h
diff --git a/cc/input/scrollbar_animation_controller_thinning.h b/cc/input/scrollbar_animation_controller_thinning.h
index d8be95f8481ce90da96ae3b76fe6f25103f6d98d..8460b057b204240e9c3e75d54ccaba7ddaf958a2 100644
--- a/cc/input/scrollbar_animation_controller_thinning.h
+++ b/cc/input/scrollbar_animation_controller_thinning.h
@@ -35,6 +35,9 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
 
   void DidScrollUpdate(bool on_resize) override;
+
+  void DidCaptureScrollbarBegin() override;
+  void DidCaptureScrollbarEnd() override;
   void DidMouseMoveOffScrollbar() override;
   void DidMouseMoveNear(float distance) override;
 
@@ -60,6 +63,7 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
   void ApplyOpacityAndThumbThicknessScale(float opacity,
                                           float thumb_thickness_scale);
 
+  bool captured_;
   bool mouse_is_over_scrollbar_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.