Add support for mojo::TypeConverter to control how convenient conversion should be.

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

Index: mojo/public/cpp/bindings/tests/sample_service_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/sample_service_unittest.cc b/mojo/public/cpp/bindings/tests/sample_service_unittest.cc
index 75d0d9b3e24d0b9b45fbe4e618fb53c5b211b4c3..9cc72f603792da6926ac590483d71d6c6ef29bfd 100644
--- a/mojo/public/cpp/bindings/tests/sample_service_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/sample_service_unittest.cc
@@ -21,6 +21,8 @@ class TypeConverter<sample::Bar, int32_t> {
            static_cast<int32_t>(bar.beta()) << 8 |
            static_cast<int32_t>(bar.gamma());
   }
+
+  MOJO_ALLOW_IMPLICIT_TYPE_CONVERSION();
 };
 
 }  // namespace mojo