[bindings] Implement an ExperimentEnabled IDL extended attribute.

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

Index: third_party/WebKit/Source/bindings/templates/interface.cpp
diff --git a/third_party/WebKit/Source/bindings/templates/interface.cpp b/third_party/WebKit/Source/bindings/templates/interface.cpp
index a44d2d110b06e0dbfd8930d396d3763e9fa062da..beef4eb70085733b75afcfb2a5276fa48f3de1f8 100644
--- a/third_party/WebKit/Source/bindings/templates/interface.cpp
+++ b/third_party/WebKit/Source/bindings/templates/interface.cpp
@@ -903,10 +903,14 @@ 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 %}