Improve TreeTypeAdapter thread-safety (google#1976)

* Improve TreeTypeAdapter thread-safety

Gson claims to be thread-safe so TreeTypeAdapter.delegate() might be
called by multiple threads. To guarantee that each thread sees a fully
constructed `delegate`, the field has to be `volatile`.

* Improve TreeTypeAdapter thread race comment

gson/src/main/java/com/google/gson/internal/bind/TreeTypeAdapter.java

@@ -47,7 +47,7 @@ public final class TreeTypeAdapter<T> extends TypeAdapter<T> {
   private final GsonContextImpl context = new GsonContextImpl();
 
   /** The delegate is lazily created because it may not be needed, and creating it may fail. */
-  private TypeAdapter<T> delegate;
+  private volatile TypeAdapter<T> delegate;
 
   public TreeTypeAdapter(JsonSerializer<T> serializer, JsonDeserializer<T> deserializer,
       Gson gson, TypeToken<T> typeToken, TypeAdapterFactory skipPast) {
@@ -83,6 +83,7 @@ public TreeTypeAdapter(JsonSerializer<T> serializer, JsonDeserializer<T> deseria
   }
 
   private TypeAdapter<T> delegate() {
+    // A race might lead to `delegate` being assigned by multiple threads but the last assignment will stick
     TypeAdapter<T> d = delegate;
     return d != null
         ? d