* 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
const std::string& message) override;
  void SendJsonRequest(const DispatchCallback& callback,
                       const std::string& browser_id,
                       const std::string& url) override;
  void GetPreferences(const DispatchCallback& callback) override;
  void SetPreference(const std::string& name,
                     const std::string& value) override;
  void RemovePreference(const std::string& name) override;
  void ClearPreferences() override;
  void RegisterExtensionsAPI(const std::string& origin,
                             const std::string& script) override;
  
  void HandleMessageFromDevToolsFrontend(const std::string& message);
  
  void DispatchProtocolMessage(content::DevToolsAgentHost* agent_host,
                               const std::string& message) override;
  void AgentHostClosed(content::DevToolsAgentHost* agent_host) override;
  
  void RenderFrameHostChanged(content::RenderFrameHost* old_host,
                              content::RenderFrameHost* new_host) override;
  void WebContentsDestroyed() override;
  void OnWebContentsFocused(
      content::RenderWidgetHost* render_widget_host) override;
  void ReadyToCommitNavigation(
      content::NavigationHandle* navigation_handle) override;
  void DidFinishNavigation(
      content::NavigationHandle* navigation_handle) override;
  
  bool DidAddMessageToConsole(content::WebContents* source,
                              int32_t level,