[bindings] Support is_explicit_nullable for method arguments.

Source/bindings/templates/methods.cpp

 {% from 'conversions.cpp' import v8_value_to_local_cpp_value %}
 
 
 {##############################################################################}
 {% macro generate_method(method, world_suffix) %}
 {% filter conditional(method.conditional_string) %}
 {% if method.returns_promise and method.has_exception_state %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}Promise(const v8::FunctionCallbackInfo<v8::Value>& info, ExceptionState& exceptionState)
 {% else %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}(const v8::FunctionCallbackInfo<v8::Value>& info)
@@ skipping 160 matching lines @@
 {% elif argument.is_dictionary %}
 {% if not argument.use_permissive_dictionary_conversion %}
 {# Dictionaries must have type Undefined, Null or Object:
 http://heycam.github.io/webidl/#es-dictionary #}
 if (!isUndefinedOrNull(info[{{argument.index}}]) && !info[{{argument.index}}]->IsObject()) {
     {{throw_type_error(method, '"parameter %s (\'%s\') is not an object."' %
                                (argument.index + 1, argument.name)) | indent}}
 }
 {% endif %}{# not argument.use_permissive_dictionary_conversion #}
 {{v8_value_to_local_cpp_value(argument)}}
+{% elif argument.is_explicit_nullable %}
+if (!isUndefinedOrNull(info[{{argument.index}}])) {
+    {{v8_value_to_local_cpp_value(argument) | indent}}
+}
 {% else %}{# argument.is_nullable #}
 {{v8_value_to_local_cpp_value(argument)}}
 {% endif %}{# argument.is_nullable #}
 {# Type checking, possibly throw a TypeError, per:
    http://www.w3.org/TR/WebIDL/#es-type-mapping #}
 {% if argument.has_type_checking_interface and not argument.is_variadic_wrapper_type %}
 {# Type checking for wrapper interface types (if interface not implemented,
    throw a TypeError), per http://www.w3.org/TR/WebIDL/#es-interface
    Note: for variadic arguments, the type checking is done for each matched
    argument instead; see argument.is_variadic_wrapper_type code-path above. #}
@@ skipping 466 matching lines @@
                               if method.overloads else
                               method.runtime_enabled_function) %}
     prototypeObject->Set(v8AtomicString(isolate, "{{method.name}}"), v8::FunctionTemplate::New(isolate, {{cpp_class_or_partial}}V8Internal::{{method.name}}MethodCallback, v8Undefined(), defaultSignature, {{method.number_of_required_arguments}})->GetFunction());
     {% endfilter %}{# runtime_enabled() #}
     {% endfilter %}{# exposed() #}
     {% endfilter %}{# per_context_enabled() #}
     {% endfor %}
     {% endif %}
 }
 {%- endmacro %}