* 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.cc
d->RegisterHandler("upgradeDraggedFileSystemPermissions",
                     &Delegate::UpgradeDraggedFileSystemPermissions, delegate);
  d->RegisterHandler("indexPath", &Delegate::IndexPath, delegate);
  d->RegisterHandlerWithCallback("loadNetworkResource",
                                 &Delegate::LoadNetworkResource, delegate);
  d->RegisterHandler("stopIndexing", &Delegate::StopIndexing, delegate);
  d->RegisterHandler("searchInPath", &Delegate::SearchInPath, delegate);
  d->RegisterHandler("setWhitelistedShortcuts",
                     &Delegate::SetWhitelistedShortcuts, delegate);
  d->RegisterHandler("zoomIn", &Delegate::ZoomIn, delegate);
  d->RegisterHandler("zoomOut", &Delegate::ZoomOut, delegate);
  d->RegisterHandler("resetZoom", &Delegate::ResetZoom, delegate);
  d->RegisterHandler("setDevicesUpdatesEnabled",
                     &Delegate::SetDevicesUpdatesEnabled, delegate);
  d->RegisterHandler("dispatchProtocolMessage",
                     &Delegate::DispatchProtocolMessageFromDevToolsFrontend,
                     delegate);
  d->RegisterHandlerWithCallback("sendJsonRequest", &Delegate::SendJsonRequest,
                                 delegate);
  d->RegisterHandlerWithCallback("getPreferences", &Delegate::GetPreferences,
                                 delegate);
  d->RegisterHandler("setPreference", &Delegate::SetPreference, delegate);
  d->RegisterHandler("removePreference", &Delegate::RemovePreference, delegate);
  d->RegisterHandler("clearPreferences", &Delegate::ClearPreferences, delegate);
  d->RegisterHandler("registerExtensionsAPI", &Delegate::RegisterExtensionsAPI,
                     delegate);
  return d;
}
}