* 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
#ifndef BRIGHTRAY_BROWSER_DEVTOOLS_EMBEDDER_MESSAGE_DISPATCHER_H_
#define BRIGHTRAY_BROWSER_DEVTOOLS_EMBEDDER_MESSAGE_DISPATCHER_H_
#include <map>
#include <string>
#include "base/callback.h"
#include "ui/gfx/geometry/insets.h"
#include "ui/gfx/geometry/rect.h"
#include "ui/gfx/geometry/size.h"
namespace base {
class ListValue;
class Value;
}  
namespace brightray {
/**
 * Dispatcher for messages sent from the DevTools frontend running in an
 * isolated renderer (on chrome-devtools://) to the embedder in the browser.
 *
 * The messages are sent via InspectorFrontendHost.sendMessageToEmbedder method.
 */
class DevToolsEmbedderMessageDispatcher {
 public:
  class Delegate {
   public:
    using DispatchCallback = base::Callback<void(const base::Value*)>;
    virtual ~Delegate() {}