Convert Runtime.GetClass to use GCHandle.

runtime/delegates.t4

@@ -89,7 +89,7 @@
 			OnlyDynamicUsage = true,
 		},
 
-		new XDelegate ("MonoObject *", "IntPtr", "xamarin_get_class",
+		new XDelegate ("GCHandle->MonoObject *", "IntPtr", "xamarin_get_class",
 			"Class", "IntPtr", "ptr"
 		) {
 			WrappedManagedFunction = "GetClass",

src/ObjCRuntime/Runtime.cs

@@ -628,7 +628,7 @@ namespace ObjCRuntime {
 
 		static IntPtr GetClass (IntPtr klass)
 		{
-			return ObjectWrapper.Convert (new Class (klass));
+			return AllocGCHandle (new Class (klass));
 		}
 
 		static IntPtr GetSelector (IntPtr sel)