* 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
virtual void ActivateWindow() = 0;
    virtual void CloseWindow() = 0;
    virtual void LoadCompleted() = 0;
    virtual void SetInspectedPageBounds(const gfx::Rect& rect) = 0;
    virtual void InspectElementCompleted() = 0;
    virtual void InspectedURLChanged(const std::string& url) = 0;
    virtual void SetIsDocked(const DispatchCallback& callback,
                             bool is_docked) = 0;
    virtual void OpenInNewTab(const std::string& url) = 0;
    virtual void SaveToFile(const std::string& url,
                            const std::string& content,
                            bool save_as) = 0;
    virtual void AppendToFile(const std::string& url,
                              const std::string& content) = 0;
    virtual void RequestFileSystems() = 0;
    virtual void AddFileSystem(const std::string& file_system_path) = 0;
    virtual void RemoveFileSystem(const std::string& file_system_path) = 0;
    virtual void UpgradeDraggedFileSystemPermissions(
        const std::string& file_system_url) = 0;
    virtual void IndexPath(int index_request_id,
                           const std::string& file_system_path) = 0;
    virtual void StopIndexing(int index_request_id) = 0;
    virtual void LoadNetworkResource(const DispatchCallback& callback,
                                     const std::string& url,
                                     const std::string& headers,
                                     int stream_id) = 0;
    virtual void SearchInPath(int search_request_id,
                              const std::string& file_system_path,