Reduce magic numbers in FrugalList.cs (#10204)

Author: lindexi

src/Microsoft.DotNet.Wpf/src/Shared/MS/Utility/FrugalList.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
@@ -340,7 +340,7 @@ public void Promote(SingleItemList<T> oldList)
 
         public override T[] ToArray()
         {
-            T[] array = new T[1];
+            T[] array = new T[SIZE];
             array[0] = _loneEntry;
             return array;
         }