Do not allow callers of Name.getRefs() to modify the refs list

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=232578832

src/com/google/javascript/jscomp/GlobalNamespace.java

@@ -1327,7 +1327,7 @@ void removeRef(Ref ref) {
     }
 
     List<Ref> getRefs() {
-      return refs == null ? ImmutableList.of() : refs;
+      return refs == null ? ImmutableList.of() : Collections.unmodifiableList(refs);
     }
 
     private void addRefInternal(Ref ref) {