History: make backing array readonly

GitOrigin-RevId: 33c1bfb915b4f2fb80c3bb1f4b88ef924a45363d

Shared/History.cs

@@ -17,7 +17,7 @@ namespace SemiColinGames {
   class History<T> {
 
     // Backing store for the History's items.
-    private T[] items;
+    private readonly T[] items;
     // Points at the most-recently-inserted item.
     private int idx = 0;