Implement overlay scrollbar fade out for non-composited scrollers.

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 87 matching lines @@
   float currentPos() const override { return m_currentPos; }
   int visibleSize() const override { return m_visibleSize; }
   int totalSize() const override { return m_totalSize; }
   int maximum() const override { return m_totalSize - m_visibleSize; }
   ScrollbarControlSize controlSize() const override { return m_controlSize; }
 
   ScrollbarPart pressedPart() const override { return m_pressedPart; }
   ScrollbarPart hoveredPart() const override { return m_hoveredPart; }
 
   void styleChanged() override {}
-  void visibilityChanged() override;
+  void setScrollbarsHidden(bool) override;

[dtapuska, 2016/11/02 16:53:21]

Personally I'd prefer if this was a positive affirmation like
setScrollbarsVisible(bool). But it could just by my mental model.

[skobes, 2016/11/02 21:02:43]

I prefer setScrollbarsHidden for consistency with other plumbing like
Settings::hideScrollbars().  Unless you're going to change it everywhere.

[bokan, 2016/11/02 22:41:57]

I preferred |hidden| to |visible| since visible is the default state and we
really are "hiding" them from hit testing. That said, I don't feel strongly
about it but if we do decide to change it should happen after this patch as
there's lots of plumbing in CC as well.
Huh, didn't even know about that setting. It "hides" scrollbars by not creating
them which is different from the flag I added, perhaps we should rename it to
"NoScrollbars" or something more like that?

   bool enabled() const override { return m_enabled; }
   void setEnabled(bool) override;
 
   int scrollbarThickness() const;
 
   // Called by the ScrollableArea when the scroll offset changes.
   // Will trigger paint invalidation if required.
   void offsetDidChange();
 
   void disconnectFromScrollableArea();
@@ skipping 137 matching lines @@
 
 DEFINE_TYPE_CASTS(Scrollbar,
                   Widget,
                   widget,
                   widget->isScrollbar(),
                   widget.isScrollbar());
 
 }  // namespace blink
 
 #endif  // Scrollbar_h