[Foundation] Move NSAttributedStringDocumentAttributes.ViewSize to shared code.

This property has also been added on macOS (where it didn't exist before), since
both documentation and headers reveal it's available there.

src/Foundation/NSAttributedString.iOS.cs

@@ -41,23 +41,6 @@ namespace Foundation {
 
 	public partial class NSAttributedStringDocumentAttributes : DictionaryContainer {
 #if !MONOMAC && !COREBUILD
-		public CGSize? ViewSize {
-			get {
-				NSObject value;
-				Dictionary.TryGetValue (NSAttributedStringDocumentAttributeKey.NSViewSizeDocumentAttribute, out value);
-				var size = value as NSValue;
-				if (size != null)
-					return size.CGSizeValue;
-				return null;
-			}
-			set {
-				if (value is null)
-					RemoveValue (NSAttributedStringDocumentAttributeKey.NSViewSizeDocumentAttribute);
-				else
-					Dictionary [NSAttributedStringDocumentAttributeKey.NSViewSizeDocumentAttribute] = NSValue.FromCGSize (value.Value);
-			}
-		}
-
 		public float? ViewZoom {
 			get {
 				return GetFloatValue (NSAttributedStringDocumentAttributeKey.NSViewZoomDocumentAttribute);

src/Foundation/NSAttributedStringDocumentAttributes.cs

@@ -157,6 +157,15 @@ namespace Foundation {
 		}
 #endif // !__MACOS__
 
+		public CGSize? ViewSize {
+			get {
+				return GetCGSizeValue (NSAttributedStringDocumentAttributeKey.NSViewSizeDocumentAttribute);
+			}
+			set {
+				SetCGSizeValue (NSAttributedStringDocumentAttributeKey.NSViewSizeDocumentAttribute, value);
+			}
+		}
+
 #if !TVOS && !WATCH
 		// documentation is unclear if an NSString or an NSUrl should be used...
 		// but providing an `NSString` throws a `NSInvalidArgumentException Reason: (null) is not a file URL`