IDL: Improve CG for attributes with [CachedAttribute]

Source/bindings/templates/attributes.cpp

 {% from 'conversions.cpp' import 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 %}
@@ skipping 66 matching lines @@
                          attribute.cpp_value)
       | indent}}
     {% endif %}
     {% if attribute.is_explicit_nullable %}
     if (isNull) {
         v8SetReturnValueNull(info);
         return;
     }
     {% endif %}
     {% if attribute.cached_attribute_validation_method %}
-    V8HiddenValue::setHiddenValue(info.GetIsolate(), holder, propertyName, {{attribute.cpp_value_to_v8_value}});
+    v8::Local<v8::Value> v8Value({{attribute.cpp_value_to_v8_value}});
+    V8HiddenValue::setHiddenValue(info.GetIsolate(), holder, propertyName, v8Value);
     {% endif %}
     {# v8SetReturnValue #}
     {% if attribute.is_keep_alive_for_gc %}
     if ({{attribute.cpp_value}} && DOMDataStore::setReturnValue{{world_suffix}}(info.GetReturnValue(), {{attribute.cpp_value}}.get()))
         return;
-    v8::Local<v8::Value> wrapper = toV8({{attribute.cpp_value}}.get(), holder, info.GetIsolate());
-    if (!wrapper.IsEmpty()) {
-        V8HiddenValue::setHiddenValue(info.GetIsolate(), holder, v8AtomicString(info.GetIsolate(), "{{attribute.name}}"), wrapper);
+    v8::Local<v8::Value> v8Value(toV8({{attribute.cpp_value}}.get(), holder, info.GetIsolate()));
+    if (!v8Value.IsEmpty()) {
+        V8HiddenValue::setHiddenValue(info.GetIsolate(), holder, v8AtomicString(info.GetIsolate(), "{{attribute.name}}"), v8Value);
         {{attribute.v8_set_return_value}};
     }
     {% elif world_suffix %}
     {{attribute.v8_set_return_value_for_main_world}};
     {% else %}
     {% if attribute.use_output_parameter_for_result %}
     {{attribute.cpp_type}} result;
     {{attribute.cpp_value}};
     {% endif %}
     {{attribute.v8_set_return_value}};
@@ skipping 342 matching lines @@
        access_control,
        property_attribute,
        only_exposed_to_private_script,
    ] %}
 {% if not attribute.is_expose_js_accessors %}
 {% set attribute_configuration_list = attribute_configuration_list
                                     + [on_prototype] %}
 {% endif %}
 {{'{'}}{{attribute_configuration_list | join(', ')}}{{'}'}}
 {%- endmacro %}