WIP bindings: Expand usage of NativeValueTraits.

third_party/WebKit/Source/bindings/templates/union_container.cpp.tmpl

 {% from 'utilities.cpp.tmpl' import declare_enum_validation_variable %}
 {% from 'utilities.cpp.tmpl' import v8_value_to_local_cpp_value %}
 {#############################################################################}
 {% macro assign_and_return_if_hasinstance(member) %}
 {% if member.is_array_buffer_or_view_type %}
 if (v8Value->Is{{member.type_name}}()) {
 {% else %}
 if (V8{{member.type_name}}::hasInstance(v8Value, isolate)) {
 {% endif %}
   {{member.cpp_local_type}} cppValue = V8{{member.type_name}}::toImpl(v8::Local<v8::Object>::Cast(v8Value));
@@ skipping 180 matching lines @@
       return {{member.cpp_value_to_v8_value}};
     {% endfor %}
     default:
       NOTREACHED();
   }
   return v8::Local<v8::Value>();
 }
 
 {{cpp_class}} NativeValueTraits<{{cpp_class}}>::nativeValue(v8::Isolate* isolate, v8::Local<v8::Value> value, ExceptionState& exceptionState) {
   {{cpp_class}} impl;
-  {{v8_class}}::toImpl(isolate, value, impl, UnionTypeConversionMode::NotNullable, exceptionState);
+  {{v8_class}}::toImpl(isolate, value, impl, UnionTypeConversionMode::{% if includes_nullable_type %}Nullable{% else %}NotNullable{% endif %}, exceptionState);
   return impl;
 }
 
 }  // namespace blink
 
 {% endfilter %}{# format_blink_cpp_source_code #}