Scaffold IScrollAnchorProvider on ScrollViewer.

It doesn't do anything yet, but `ItemsPresenter` requires `ScrollViewer` to implement this interface.

src/Avalonia.Controls/Repeaters/ViewportManager.cs

@@ -365,7 +365,7 @@ namespace Avalonia.Controls.Repeaters
         public void ResetScrollers()
         {
             _scroller = null;
-            _effectiveViewportChangedRevoker.Dispose();
+            _effectiveViewportChangedRevoker?.Dispose();
             _effectiveViewportChangedRevoker = null;
             _ensuredScroller = false;
         }

src/Avalonia.Controls/ScrollViewer.cs

@@ -11,7 +11,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control scrolls its content if the content is bigger than the space available.
     /// </summary>
-    public class ScrollViewer : ContentControl, IScrollable
+    public class ScrollViewer : ContentControl, IScrollable, IScrollAnchorProvider
     {
         /// <summary>
         /// Defines the <see cref="CanHorizontallyScroll"/> property.
@@ -333,6 +333,9 @@ namespace Avalonia.Controls
             get { return _viewport.Height; }
         }
 
+        /// <inheritdoc/>
+        IControl IScrollAnchorProvider.CurrentAnchor => null; // TODO: Implement
+
         /// <summary>
         /// Gets the value of the HorizontalScrollBarVisibility attached property.
         /// </summary>
@@ -373,6 +376,16 @@ namespace Avalonia.Controls
             control.SetValue(VerticalScrollBarVisibilityProperty, value);
         }
 
+        void IScrollAnchorProvider.RegisterAnchorCandidate(IControl element)
+        {
+            // TODO: Implement
+        }
+
+        void IScrollAnchorProvider.UnregisterAnchorCandidate(IControl element)
+        {
+            // TODO: Implement
+        }
+
         internal static Vector CoerceOffset(Size extent, Size viewport, Vector offset)
         {
             var maxX = Math.Max(extent.Width - viewport.Width, 0);