* 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.h
const std::string& query) = 0;
    virtual void SetWhitelistedShortcuts(const std::string& message) = 0;
    virtual void ZoomIn() = 0;
    virtual void ZoomOut() = 0;
    virtual void ResetZoom() = 0;
    virtual void SetDevicesUpdatesEnabled(bool enabled) = 0;
    virtual void DispatchProtocolMessageFromDevToolsFrontend(
        const std::string& message) = 0;
    virtual void SendJsonRequest(const DispatchCallback& callback,
                                 const std::string& browser_id,
                                 const std::string& url) = 0;
    virtual void GetPreferences(const DispatchCallback& callback) = 0;
    virtual void SetPreference(const std::string& name,
                               const std::string& value) = 0;
    virtual void RemovePreference(const std::string& name) = 0;
    virtual void ClearPreferences() = 0;
    virtual void RegisterExtensionsAPI(const std::string& origin,
                                       const std::string& script) = 0;
  };
  using DispatchCallback = Delegate::DispatchCallback;
  virtual ~DevToolsEmbedderMessageDispatcher() {}
  virtual bool Dispatch(const DispatchCallback& callback,
                        const std::string& method,
                        const base::ListValue* params) = 0;
  static DevToolsEmbedderMessageDispatcher* CreateForDevToolsFrontend(
      Delegate* delegate);
};
}  
#endif