bindings: Pass is_null flag to attribute setters when they are nullable

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

 {% from 'utilities.cpp.tmpl' import declare_enum_validation_variable, v8_value_to_local_cpp_value %}
 
 {##############################################################################}
 {% macro attribute_getter(attribute, world_suffix) %}
 static void {{attribute.name}}AttributeGetter{{world_suffix}}(
 {%- if attribute.is_data_type_property %}
 const v8::PropertyCallbackInfo<v8::Value>& info
 {%- else %}
 const v8::FunctionCallbackInfo<v8::Value>& info
 {%- endif %}) {
@@ skipping 378 matching lines @@
   {% if attribute.is_static %}
   ScriptState* scriptState = ScriptState::ForFunctionObject(info);
   {% else %}
   ScriptState* scriptState = ScriptState::ForReceiverObject(info);
   {% endif %}
   {% endif %}
 
   {% if attribute.is_replaceable %}
   v8::Local<v8::String> propertyName = V8AtomicString(isolate, "{{attribute.name}}");
   {% endif %}
+  {% if attribute.is_explicit_nullable %}
+  bool isNull = IsUndefinedOrNull(v8Value);
+  {% endif %}
   {{attribute.cpp_setter}};
 
   {% if attribute.cached_attribute_validation_method %}
   // [CachedAttribute]
   // Invalidate the cached value.
   V8PrivateProperty::GetSymbol(
       isolate, "{{cpp_class}}#{{attribute.name.capitalize()}}")
       .DeleteProperty(holder, v8::Undefined(isolate));
   {% endif %}
 }
@@ skipping 106 matching lines @@
 static const V8DOMConfiguration::AccessorConfiguration accessorConfiguration[] = {
   {{attribute_configuration(attribute)}}
 };
 for (const auto& accessorConfig : accessorConfiguration)
   V8DOMConfiguration::InstallAccessor(isolate, world, v8::Local<v8::Object>(), prototypeObject, interfaceObject, signature, accessorConfig);
 {% endfilter %}{# runtime_enabled #}
 {% endfilter %}{# secure_context #}
 {% endfilter %}{# exposed #}
 {% endfor %}
 {% endmacro %}