revert scroll_position.cs

com.unity.uiwidgets/Runtime/widgets/scroll_position.cs

             this.keepScrollOffset = keepScrollOffset;
             this.debugLabel = debugLabel;
             _coordinator = coordinator;
-
             if (oldPosition != null) {
                 absorb(oldPosition);
             }
         internal readonly object _coordinator;
 
         public float minScrollExtent {
-            get { return _minScrollExtent; }
+            get { return _minScrollExtent.Value; }
-        float _minScrollExtent;
+        float? _minScrollExtent;
-            get { return _maxScrollExtent; }
+            get { return _maxScrollExtent.Value; }
-        float _maxScrollExtent;
+        float? _maxScrollExtent;
+
+        public bool hasMinScrollExtent {
+            get { return _minScrollExtent != null; }
+        }
+        public bool hasMaxScrollExtent {
+            get { return _maxScrollExtent != null; }
+        }
+        
         public override float pixels {
             get {
                 D.assert(_pixels != null);
         internal float? _pixels;
 
         public float viewportDimension {
-            get { return _viewportDimension; }
+            get { return _viewportDimension.Value; }
-        float _viewportDimension;
+        float? _viewportDimension;
 
         public bool haveDimensions {
             get { return _haveDimensions; }