Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/public/platform/WebThemeEngine.h

 /*
  * Copyright (C) 2010 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 147 matching lines @@
   // like vertical scrollbar thumbs, the width will be the required width of
   // the track while the height will be the minimum height.
   virtual WebSize getSize(Part) { return WebSize(); }
 
   struct ScrollbarStyle {
     int thumbThickness;
     int scrollbarMargin;
     WebColor color;
   };
 
+  virtual double getOverlayScrollbarFadeOutDelaySeconds() { return 0.0; }

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

Could these just be fields in ScrollbarStyle and reuse getOverlayScrollbarStyle?

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

Yup, done. I also added a TODO to set those members for Android so we can get
the same behavior for non-composited scrollers in Android. That'll require some
plumbing to move the constants into a single place so I'll leave that for a
future patch.

+  virtual double getOverlayScrollbarFadeOutDurationSeconds() { return 0.0; }
+
   // Gets the overlay scrollbar style. Used for mobile theme.
   virtual void getOverlayScrollbarStyle(ScrollbarStyle*) {}
 
   // Paint the given the given theme part.
   virtual void paint(WebCanvas*,
                      Part,
                      State,
                      const WebRect&,
                      const ExtraParams*) {}
 };
 
 }  // namespace blink
 
 #endif