* Electron Version: 3.0.0-beta.1
* Operating System (Platform and Version): macOS 10.13.5
* Last known working Electron version: 2.0.3

**Expected Behavior**
Dev tools extension tabs should appear after being added with the `BrowserWindow.addDevToolsExtension` API. I specifically tested the React and Redux extensions. I can tell the React extension is getting installed but not being shown because the usual
> Download the React DevTools for a better development experience

warning is absent.

**Actual behavior**
Dev tools extension tabs do not appear.

**To Reproduce**
Manually obtain the extension paths following the instructions here:
https://electronjs.org/docs/tutorial/devtools-extension
and supply them to `addDevToolsExtensions`.

Alternately, use [electron-devtools-installer](https://github.com/MarshallOfSound/electron-devtools-installer), which I have a repro case for using just Redux here:
https://github.com/ajmacd/electron-quick-start-typescript

Working with 2.0.3:
```sh
$ git clone https://github.com/ajmacd/electron-quick-start-typescript -b 2.0.3-devtools-working 
$ cd electron-quick-start-typescript
$ npm install
$ npm start
```

Not working with 3.0.0-beta.1:
```sh
$ git checkout 3.0.0-beta.1-devtools-not-working
$ npm install
$ npm start
```
brightray/browser/devtools_embedder_message_dispatcher.cc
bool Dispatch(const DispatchCallback& callback,
                const std::string& method,
                const base::ListValue* params) override {
    auto it = handlers_.find(method);
    return it != handlers_.end() && it->second.Run(callback, *params);
  }
  template <typename... As>
  void RegisterHandler(const std::string& method,
                       void (Delegate::*handler)(As...),
                       Delegate* delegate) {
    handlers_[method] =
        base::Bind(&ParseAndHandle<As...>,
                   base::Bind(handler, base::Unretained(delegate)));
  }
  template <typename... As>
  void RegisterHandlerWithCallback(
      const std::string& method,
      void (Delegate::*handler)(const DispatchCallback&, As...),
      Delegate* delegate) {
    handlers_[method] =
        base::Bind(&ParseAndHandleWithCallback<As...>,
                   base::Bind(handler, base::Unretained(delegate)));
  }
 private:
  using Handler =
      base::Callback<bool(const DispatchCallback&, const base::ListValue&)>;
  using HandlerMap = std::map<std::string, Handler>;
  HandlerMap handlers_;
};
DevToolsEmbedderMessageDispatcher*