Overlay scrollbars are painted onload.

cc/input/scrollbar_animation_controller_thinning.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 38 matching lines @@
   void RunAnimationFrame(float progress) override;
 
  private:
   // Describes whether the current animation should INCREASE (darken / thicken)
   // a bar or DECREASE it (lighten / thin).
   enum AnimationChange { NONE, INCREASE, DECREASE };
   float OpacityAtAnimationProgress(float progress);
   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);
 
   bool mouse_is_over_scrollbar_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;
   // Are we darkening or lightening the bars.
   AnimationChange opacity_change_;
   // How close should the mouse be to the scrollbar before we thicken it.
   float mouse_move_distance_to_trigger_animation_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerThinning);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_