[bindings] Implement an ExperimentEnabled IDL extended attribute.

third_party/WebKit/Source/bindings/templates/interface.cpp

 {% extends 'interface_base.cpp' %}
 
 
 {##############################################################################}
 {% block indexed_property_getter %}
 {% if indexed_property_getter and not indexed_property_getter.is_custom %}
 {% set getter = indexed_property_getter %}
 static void indexedPropertyGetter(uint32_t index, const v8::PropertyCallbackInfo<v8::Value>& info)
 {
     {{cpp_class}}* impl = {{v8_class}}::toImpl(info.Holder());
@@ skipping 885 matching lines @@
 
 
 {##############################################################################}
 {% macro install_unscopeables() %}
 v8::Local<v8::Name> unscopablesSymbol(v8::Symbol::GetUnscopables(isolate));
 v8::Local<v8::Object> unscopeables;
 if (v8CallBoolean(prototypeObject->HasOwnProperty(context, unscopablesSymbol)))
     unscopeables = prototypeObject->Get(context, unscopablesSymbol).ToLocalChecked().As<v8::Object>();
 else
     unscopeables = v8::Object::New(isolate);
-{% for name, runtime_enabled_function in unscopeables %}
+String errorMessage;
+ALLOW_UNUSED_LOCAL(errorMessage);
+{% for name, runtime_enabled_function, experimental_api_name in unscopeables %}
 {% filter runtime_enabled(runtime_enabled_function) %}
+{% filter experiment_enabled(experimental_api_name, "errorMessage") %}
 unscopeables->CreateDataProperty(context, v8AtomicString(isolate, "{{name}}"), v8::True(isolate)).FromJust();
 {% endfilter %}
+{% endfilter %}
 {% endfor %}
 prototypeObject->CreateDataProperty(context, unscopablesSymbol, unscopeables).FromJust();
 {% endmacro %}
 
 
 {##############################################################################}
 {% macro install_conditionally_enabled_attributes_on_prototype() %}
 {% from 'attributes.cpp' import attribute_configuration with context %}
 ExecutionContext* executionContext = toExecutionContext(context);
 v8::Local<v8::Signature> signature = v8::Signature::New(isolate, interfaceTemplate);
@@ skipping 58 matching lines @@
 
 {% for method in methods if method.overloads and method.overloads.has_partial_overloads %}
 void {{v8_class}}::register{{method.name | blink_capitalize}}MethodForPartialInterface(void (*method)(const v8::FunctionCallbackInfo<v8::Value>&))
 {
     {{cpp_class}}V8Internal::{{method.name}}MethodForPartialInterface = method;
 }
 
 {% endfor %}
 {% endif %}
 {% endblock %}