Implement overlay scrollbar fade out for non-composited scrollers.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
index 7aefff29abf49721b30570ab40baa1e7a725eb7c..5f36438de4797e6000797c9cbed2219d2c383c4f 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
@@ -294,6 +294,8 @@ class CORE_EXPORT PaintLayerScrollableArea final
   ScrollBehavior scrollBehaviorStyle() const override;
   CompositorAnimationTimeline* compositorAnimationTimeline() const override;
 
+  void visibleSizeChanged();
+
   // FIXME: We shouldn't allow access to m_overflowRect outside this class.
   LayoutRect overflowRect() const { return m_overflowRect; }
 
@@ -320,8 +322,6 @@ class CORE_EXPORT PaintLayerScrollableArea final
   void updateAfterLayout();
   void clampScrollOffsetsAfterLayout();
 
-  void didChangeScrollbarsHidden() override;
-
   void updateAfterStyleChange(const ComputedStyle*);
   void updateAfterOverflowRecalc();
 
@@ -480,8 +480,6 @@ class CORE_EXPORT PaintLayerScrollableArea final
  private:
   explicit PaintLayerScrollableArea(PaintLayer&);
 
-  void updateScrollbarsEnabledState();
-
   bool hasHorizontalOverflow() const;
   bool hasVerticalOverflow() const;
   bool hasScrollableHorizontalOverflow() const;