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 145 matching lines @@
 
   // Gets the size of the given theme part. For variable sized items
   // 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;
+    double fadeOutDelaySeconds;
+    double fadeOutDurationSeconds;
   };
 
-  // Gets the overlay scrollbar style. Used for mobile theme.
-  virtual void getOverlayScrollbarStyle(ScrollbarStyle*) {}
+  // Gets the overlay scrollbar style. Not used on Mac.
+  virtual void getOverlayScrollbarStyle(ScrollbarStyle* style) {
+    // Disable overlay scrollbar fade out (for non-composited scrollers) unless
+    // explicitly enabled by the implementing child class. NOTE: these values
+    // aren't used to control Mac fade out - that happens in ScrollAnimatorMac.
+    style->fadeOutDelaySeconds = 0.0;
+    style->fadeOutDurationSeconds = 0.0;
+    // The other fields in this struct are used only on Android to draw solid
+    // color scrollbars. On other platforms the scrollbars are painted in
+    // NativeTheme so these fields are unused in non-Android WebThemeEngines.
+  }
 
   // Paint the given the given theme part.
   virtual void paint(WebCanvas*,
                      Part,
                      State,
                      const WebRect&,
                      const ExtraParams*) {}
 };
 
 }  // namespace blink
 
 #endif