Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeOverlay.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 35 matching lines @@
                         Color);
   ~ScrollbarThemeOverlay() override {}
 
   bool ShouldRepaintAllPartsOnInvalidation() const override;
 
   ScrollbarPart InvalidateOnThumbPositionChange(
       const ScrollbarThemeClient&,
       float old_position,
       float new_position) const override;
 
-  ScrollbarPart InvalidateOnEnabledChange() const override;
-
   int ScrollbarThickness(ScrollbarControlSize) override;
   int ScrollbarMargin() const override;
   bool UsesOverlayScrollbars() const override;
   double OverlayScrollbarFadeOutDelaySeconds() const override;
   double OverlayScrollbarFadeOutDurationSeconds() const override;
 
   int ThumbPosition(const ScrollbarThemeClient&,
                     float scroll_position) override;
   int ThumbLength(const ScrollbarThemeClient&) override;
 
@@ skipping 26 matching lines @@
   int thumb_thickness_;
   int scrollbar_margin_;
   HitTestBehavior allow_hit_test_;
   Color color_;
   const bool use_solid_color_;
 };
 
 }  // namespace blink
 
 #endif