Update comments about scrollbar thickness.

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

 /*
  * Copyright (C) 2004, 2006 Apple Computer, 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 92 matching lines @@
   ScrollbarPart pressedPart() const override { return m_pressedPart; }
   ScrollbarPart hoveredPart() const override { return m_hoveredPart; }
 
   void styleChanged() override {}
   void setScrollbarsHidden(bool) override;
   bool enabled() const override { return m_enabled; }
   void setEnabled(bool) override;
 
   // This returns device-scale-factor-aware pixel value.
   // e.g. 15 in dsf=1.0, 30 in dsf=2.0.
+  // This returns 0 for overlay scrollbars.
   // See also ScrolbarTheme::scrollbatThickness().
   int scrollbarThickness() const;
 
   // Called by the ScrollableArea when the scroll offset changes.
   // Will trigger paint invalidation if required.
   void offsetDidChange();
 
   void disconnectFromScrollableArea();
   ScrollableArea* getScrollableArea() const { return m_scrollableArea; }
 
@@ skipping 135 matching lines @@
 
 DEFINE_TYPE_CASTS(Scrollbar,
                   Widget,
                   widget,
                   widget->isScrollbar(),
                   widget.isScrollbar());
 
 }  // namespace blink
 
 #endif  // Scrollbar_h