[WPF] Fix ScrollView sizing issue

Use the default size defined by XWT when no specific size
has been set.

Xwt.WPF/Xwt.WPFBackend/ScrollViewBackend.cs

@@ -51,6 +51,22 @@ namespace Xwt.WPFBackend
 			set { ScrollViewer.HorizontalScrollBarVisibility = GetScrollVisibility (value); }
 		}
 
+		protected override double DefaultNaturalHeight
+		{
+			get
+			{
+				return VerticalScrollPolicy != ScrollPolicy.Never ? - 1 : -2;
+			}
+		}
+
+		protected override double DefaultNaturalWidth
+		{
+			get
+			{
+				return HorizontalScrollPolicy != ScrollPolicy.Never ? -1 : -2;
+			}
+		}
+
 		private bool borderVisible = true;
 		public bool BorderVisible
 		{
@@ -85,6 +101,7 @@ namespace Xwt.WPFBackend
 			ScrollAdjustmentBackend vbackend = null, hbackend = null;
 			var widget = (WidgetBackend)child;
 
+
 			if (widget.EventSink.SupportsCustomScrolling ()) {
 				vbackend = new ScrollAdjustmentBackend ();
 				hbackend = new ScrollAdjustmentBackend ();