Mojo C++ bindings: support setting default value for fields whose types are typemapped enums.

mojo/public/cpp/bindings/tests/struct_traits_unittest.cc

Index: mojo/public/cpp/bindings/tests/struct_traits_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/struct_traits_unittest.cc b/mojo/public/cpp/bindings/tests/struct_traits_unittest.cc
index 7e4bcd12f71c02af13096133462ced48d0a2797d..77122c975d09f203b0b662d04c32b65d001d8f6c 100644
--- a/mojo/public/cpp/bindings/tests/struct_traits_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/struct_traits_unittest.cc
@@ -550,5 +550,10 @@ TEST_F(StructTraitsTest, EchoUnionWithTraits) {
   }
 }
 
+TEST_F(StructTraitsTest, DefaultValueOfEnumWithTraits) {
+  auto container = EnumWithTraitsContainer::New();
+  EXPECT_EQ(EnumWithTraitsImpl::CUSTOM_VALUE_1, container->f_field);
+}
+
 }  // namespace test
 }  // namespace mojo