[bindings] Implement an ExperimentEnabled IDL extended attribute.

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

 {% macro v8_value_to_local_cpp_value(thing) %}
 {# This indirection is just to avoid spurious white-space lines. #}
 {{generate_v8_value_to_local_cpp_value(thing) | trim}}
 {%- endmacro %}
 
 
 {% macro generate_v8_value_to_local_cpp_value(thing) %}
 {% set item = thing.v8_value_to_local_cpp_value or thing %}
 {% if item.error_message %}
 /* {{item.error_message}} */
@@ skipping 35 matching lines @@
 {% set property_location_list = property_location_list + ['V8DOMConfiguration::OnInstance'] %}
 {% endif %}
 {% if member.on_prototype %}
 {% set property_location_list = property_location_list + ['V8DOMConfiguration::OnPrototype'] %}
 {% endif %}
 {% if member.on_interface %}
 {% set property_location_list = property_location_list + ['V8DOMConfiguration::OnInterface'] %}
 {% endif %}
 {{property_location_list | join(' | ')}}
 {%- endmacro %}
+
+
+{% macro check_api_experiment_internal(errorName, experiment_name) %}
+{% if experiment_name %}
+String {{errorName}};
+if (!{{experiment_name}}(executionContext, {{errorName}})) {
+     v8SetReturnValue(info, v8::Undefined(info.GetIsolate()));
+     toDocument(executionContext)->addConsoleMessage(ConsoleMessage::create(JSMessageSource, ErrorMessageLevel, {{errorName}}));
+     return;
+}
+{% endif %}
+{% endmacro %}
+
+
+{% macro check_api_experiment(member, isolate="info.GetIsolate()") -%}
+ExecutionContext* executionContext = currentExecutionContext({{isolate}});
+{{check_api_experiment_internal("errorMessage", member.api_experiment_enabled_per_interface) -}}
+{{check_api_experiment_internal("memberErrorMessage", member.api_experiment_enabled) -}}
+{% endmacro %}