Related to the RootActor conversion to protocol.js, we need to also update the WebExtensionChildActor to protocol.js. It is instantiated via (WebExtensionParentActor/webExtensionSpec).connect() via child.js

While working on this, i discovered that even without having a protocol type actor to inherit from, prototype inheritance is still not trivial. At the moment im not able to make it pass without inheriting from ChromeActor, and since it needs to do that eventually, I will probably solve this in the ChromeActor issue https://bugzilla.mozilla.org/show_bug.cgi?id=1450948