Implement python mojo bindings unions.

mojo/public/interfaces/bindings/tests/test_unions.mojom

Index: mojo/public/interfaces/bindings/tests/test_unions.mojom
diff --git a/mojo/public/interfaces/bindings/tests/test_unions.mojom b/mojo/public/interfaces/bindings/tests/test_unions.mojom
index 6e631b22afd4e55b6774f23373024582c378b2b7..e998f47f192499be86a9af485948c74a987c7ad4 100644
--- a/mojo/public/interfaces/bindings/tests/test_unions.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_unions.mojom
@@ -69,6 +69,7 @@ struct SmallStructNonNullableUnion {
 
 struct SmallObjStruct {
   ObjectUnion obj_union;
+  int8 f_int8;
 };
 
 interface SmallCache {
@@ -84,3 +85,12 @@ struct TryNonNullStruct {
   DummyStruct? nullable;
   DummyStruct non_nullable;
 };
+
+union OldUnion {
+  int8 f_int8;
+};
+
+union NewUnion {
+  int8 f_int8;
+  int16 f_int16;
+};