bindings: Supports [LenientThis] extended attributes.

Source/bindings/templates/attributes.cpp

 {% from 'conversions.cpp' import declare_enum_validation_variable, v8_value_to_local_cpp_value %}
 
 
 {##############################################################################}
 {% macro attribute_getter(attribute, world_suffix) %}
 {% filter conditional(attribute.conditional_string) %}
 static void {{attribute.name}}AttributeGetter{{world_suffix}}(
 {%- if attribute.is_expose_js_accessors %}
 const v8::FunctionCallbackInfo<v8::Value>& info
 {%- else %}
 const v8::PropertyCallbackInfo<v8::Value>& info
 {%- endif %})
 {
     {% if attribute.is_reflect and not attribute.is_url
           and attribute.idl_type == 'DOMString' and is_node
           and not attribute.is_implemented_in_private_script %}
     {% set cpp_class, v8_class = 'Element', 'V8Element' %}
     {% endif %}
     {# holder #}
     {% if not attribute.is_static %}
+    {% if attribute.is_lenient_this %}
+    v8::Local<v8::Object> holder = {{v8_class}}::findInstanceInPrototypeChain(info.This(), info.GetIsolate());
+    if (holder.IsEmpty())
+        return; // Return silently because of [LenientThis].
+    // Note that it's okay to use |holder|, but |info.Holder()| is still unsafe
+    // and must not be used.
+    {% else %}
     v8::Local<v8::Object> holder = info.Holder();
     {% endif %}
+    {% endif %}
     {# impl #}
     {% if attribute.cached_attribute_validation_method %}
     v8::Local<v8::String> propertyName = v8AtomicString(info.GetIsolate(), "{{attribute.name}}");
     {{cpp_class}}* impl = {{v8_class}}::toImpl(holder);
     if (!impl->{{attribute.cached_attribute_validation_method}}()) {
         v8::Local<v8::Value> v8Value = V8HiddenValue::getHiddenValue(info.GetIsolate(), holder, propertyName);
         if (!v8Value.IsEmpty()) {
             v8SetReturnValue(info, v8Value);
             return;
         }
@@ skipping 218 matching lines @@
             attribute.is_check_security_for_window)) %}
     {% set raise_exception = 1 %}
     {% else %}
     {% set raise_exception = 0 %}
     {% endif %}
     {# Local variables #}
     {% if (not attribute.is_static and
            not attribute.is_replaceable and
            not attribute.constructor_type) or
           raise_exception %}
+    {% if attribute.is_lenient_this %}
+    v8::Local<v8::Object> holder = {{v8_class}}::findInstanceInPrototypeChain(info.This(), info.GetIsolate());
+    if (holder.IsEmpty())
+        return; // Return silently because of [LenientThis].
+    // Note that it's okay to use |holder|, but |info.Holder()| is still unsafe
+    // and must not be used.
+    {% else %}
     v8::Local<v8::Object> holder = info.Holder();
     {% endif %}
+    {% endif %}
     {% if raise_exception %}
     ExceptionState exceptionState(ExceptionState::SetterContext, "{{attribute.name}}", "{{interface_name}}", holder, info.GetIsolate());
     {% endif %}
     {% if attribute.is_replaceable or
           attribute.constructor_type %}
     v8::Local<v8::String> propertyName = v8AtomicString(info.GetIsolate(), "{{attribute.name}}");
     {% endif %}
     {# impl #}
     {% if attribute.is_put_forwards %}
     {{cpp_class}}* proxyImpl = {{v8_class}}::toImpl(holder);
@@ skipping 235 matching lines @@
                         ' | '.join(attribute.access_control_list) %}
 {% set property_attribute = 'static_cast<v8::PropertyAttribute>(%s)' %
                             ' | '.join(attribute.property_attributes) %}
 {% set only_exposed_to_private_script =
        'V8DOMConfiguration::OnlyExposedToPrivateScript'
        if attribute.only_exposed_to_private_script else
        'V8DOMConfiguration::ExposedToAllScripts' %}
 {% set on_prototype = 'V8DOMConfiguration::OnPrototype'
        if interface_name == 'Window' and attribute.idl_type == 'EventHandler'
        else 'V8DOMConfiguration::OnInstance' %}
+{% set holder_check = 'V8DOMConfiguration::DoNotCheckHolder'
+       if attribute.is_lenient_this else 'V8DOMConfiguration::CheckHolder' %}
 {% set attribute_configuration_list = [
        '"%s"' % attribute.name,
        getter_callback,
        setter_callback,
        getter_callback_for_main_world,
        setter_callback_for_main_world,
        wrapper_type_info,
        access_control,
        property_attribute,
        only_exposed_to_private_script,
    ] %}
-{% if not attribute.is_expose_js_accessors %}
+{% if attribute.is_expose_js_accessors %}
+{% set attribute_configuration_list = attribute_configuration_list
+                                    + [holder_check] %}
+{% else %}
 {% set attribute_configuration_list = attribute_configuration_list
                                     + [on_prototype] %}
 {% endif %}
 {{'{'}}{{attribute_configuration_list | join(', ')}}{{'}'}}
 {%- endmacro %}