Overlay scrollbars are painted onload.

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..5fbec22d6fc3ebe0dcfd084e79cce5838dea437e 100644
--- a/cc/input/scrollbar_animation_controller_thinning.h
+++ b/cc/input/scrollbar_animation_controller_thinning.h
@@ -56,7 +56,9 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
   float ThumbThicknessScaleAtAnimationProgress(float progress);
   float AdjustScale(float new_value,
                     float current_value,
-                    AnimationChange animation_change);
+                    AnimationChange animation_change,
+                    float min_value,
+                    float max_value);
   void ApplyOpacityAndThumbThicknessScale(float opacity,
                                           float thumb_thickness_scale);