Copied from cpovirk's CL 147393681 to submit while cpovirk is OOO.

Fix GWT serialization broken by CL 146683890.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147488537

guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/RegularImmutableList.java

@@ -27,6 +27,7 @@
  */
 class RegularImmutableList<E> extends ForwardingImmutableList<E> {
   private final List<E> delegate;
+  E forSerialization;
 
   RegularImmutableList(List<E> delegate) {
     // TODO(cpovirk): avoid redundant unmodifiableList wrapping