Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

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

 /*
  * 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 51 matching lines @@
   return false;
 }
 
 ScrollbarPart ScrollbarThemeOverlay::InvalidateOnThumbPositionChange(
     const ScrollbarThemeClient&,
     float old_position,
     float new_position) const {
   return kNoPart;
 }
 
-ScrollbarPart ScrollbarThemeOverlay::InvalidateOnEnabledChange() const {
-  return kNoPart;
-}
-
 int ScrollbarThemeOverlay::ScrollbarThickness(
     ScrollbarControlSize control_size) {
   return thumb_thickness_ + scrollbar_margin_;
 }
 
 int ScrollbarThemeOverlay::ScrollbarMargin() const {
   return scrollbar_margin_;
 }
 
 bool ScrollbarThemeOverlay::UsesOverlayScrollbars() const {
@@ skipping 204 matching lines @@
         .height;
   }
 
   return Platform::Current()
       ->ThemeEngine()
       ->GetSize(WebThemeEngine::kPartScrollbarHorizontalThumb)
       .width;
 }
 
 }  // namespace blink