Update comments about scrollbar thickness.

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

 /*
  * Copyright (C) 2008 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 41 matching lines @@
   // which are explicitly invalidated will be repainted.
   virtual bool shouldRepaintAllPartsOnInvalidation() const { return true; }
 
   virtual void updateEnabledState(const ScrollbarThemeClient&) {}
 
   virtual bool paint(const Scrollbar&, GraphicsContext&, const CullRect&);
 
   virtual ScrollbarPart hitTest(const ScrollbarThemeClient&, const IntPoint&);
 
   // This returns a fixed value regardless of device-scale-factor.
+  // This returns thickness when scrollbar is painted.  i.e. It's not 0 even in
+  // overlay scrollbar mode.
   // See also Scrollbar::scrollbarThickness().
   virtual int scrollbarThickness(ScrollbarControlSize = RegularScrollbar) {
     return 0;
   }
   virtual int scrollbarMargin() const { return 0; }
 
   virtual WebScrollbarButtonsPlacement buttonsPlacement() const {
     return WebScrollbarButtonsPlacementSingle;
   }
 
@@ skipping 118 matching lines @@
   static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   nativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool gMockScrollbarsEnabled;
 };
 
 }  // namespace blink
 #endif