* 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
#ifndef BRIGHTRAY_BROWSER_INSPECTABLE_WEB_CONTENTS_IMPL_H_
#define BRIGHTRAY_BROWSER_INSPECTABLE_WEB_CONTENTS_IMPL_H_
#include <map>
#include <string>
#include <vector>
#include "base/memory/weak_ptr.h"
#include "brightray/browser/devtools_contents_resizing_strategy.h"
#include "brightray/browser/devtools_embedder_message_dispatcher.h"
#include "brightray/browser/inspectable_web_contents.h"
#include "content/public/browser/devtools_agent_host.h"
#include "content/public/browser/devtools_frontend_host.h"
#include "content/public/browser/web_contents_delegate.h"
#include "content/public/browser/web_contents_observer.h"
#include "net/url_request/url_fetcher_delegate.h"
#include "ui/gfx/geometry/rect.h"
class PrefService;