[DO NOT LAND] Set @@toStringTag for DOM object prototypes

Source/bindings/templates/interface_base.cpp

 {% include 'copyright_block.txt' %}
 #include "config.h"
 {% filter conditional(conditional_string) %}
 #include "{{v8_class_or_partial}}.h"
 
 {% for filename in cpp_includes if filename != '%s.h' % cpp_class_or_partial %}
 #include "{{filename}}"
 {% endfor %}
 
 namespace blink {
@@ skipping 451 matching lines @@
 
     prototypeTemplate->SetInternalFieldCount(V8Window::internalFieldCount);
     functionTemplate->SetHiddenPrototype(true);
     instanceTemplate->SetInternalFieldCount(V8Window::internalFieldCount);
     {% elif interface_name in [
            'HTMLDocument', 'DedicatedWorkerGlobalScope', 'CompositorWorkerGlobalScope',
            'SharedWorkerGlobalScope', 'ServiceWorkerGlobalScope'] %}
     functionTemplate->SetHiddenPrototype(true);
     {% endif %}
 
+    {# Willful violation of WebIDL 4.5.4 and 4.7 - implement the class string
+       for interfaces via @@toStringTag on the prototype. WebIDL requires
+       @@toStringTag instances and prototype (with 'Prototype' appended) but
+       this has performance concerns and may not be necessary for web compat.
+       https://www.w3.org/Bugs/Public/show_bug.cgi?id=28244 #}
+    prototypeTemplate->Set(v8::Symbol::GetToStringTag(isolate), v8AtomicString(isolate, "{{interface_name}}"));
+
     // Custom toString template
     functionTemplate->Set(v8AtomicString(isolate, "toString"), V8PerIsolateData::from(isolate)->toStringTemplate());
     {% endif %}
 }
 
 {% endif %}{# not is_array_buffer_or_view #}
 {% endblock %}
 {##############################################################################}
 {% block get_dom_template %}{% endblock %}
 {% block has_instance %}{% endblock %}
@@ skipping 11 matching lines @@
 {% endfor %}
 {% for attribute in attributes if attribute.is_implemented_in_private_script %}
 {{attribute_getter_implemented_in_private_script(attribute)}}
 {% if attribute.has_setter %}
 {{attribute_setter_implemented_in_private_script(attribute)}}
 {% endif %}
 {% endfor %}
 {% block partial_interface %}{% endblock %}
 } // namespace blink
 {% endfilter %}