* 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/inspectable_web_contents_impl.h
~InspectableWebContentsImpl() override;
  InspectableWebContentsView* GetView() const override;
  content::WebContents* GetWebContents() const override;
  content::WebContents* GetDevToolsWebContents() const override;
  void SetDelegate(InspectableWebContentsDelegate* delegate) override;
  InspectableWebContentsDelegate* GetDelegate() const override;
  void SetDevToolsWebContents(content::WebContents* devtools) override;
  void SetDockState(const std::string& state) override;
  void ShowDevTools() override;
  void CloseDevTools() override;
  bool IsDevToolsViewShowing() override;
  void AttachTo(scoped_refptr<content::DevToolsAgentHost>) override;
  void Detach() override;
  void CallClientFunction(const std::string& function_name,
                          const base::Value* arg1,
                          const base::Value* arg2,
                          const base::Value* arg3) override;
  void InspectElement(int x, int y) override;
  
  gfx::Rect GetDevToolsBounds() const;
  void SaveDevToolsBounds(const gfx::Rect& bounds);
  
  double GetDevToolsZoomLevel() const;
  void UpdateDevToolsZoomLevel(double level);
 private:
  
  void ActivateWindow() override;
  void CloseWindow() override;
  void LoadCompleted() override;
  void SetInspectedPageBounds(const gfx::Rect& rect) override;