* 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
DevToolsEmbedderMessageDispatcher::CreateForDevToolsFrontend(
    Delegate* delegate) {
  auto* d = new DispatcherImpl();
  d->RegisterHandler("bringToFront", &Delegate::ActivateWindow, delegate);
  d->RegisterHandler("closeWindow", &Delegate::CloseWindow, delegate);
  d->RegisterHandler("loadCompleted", &Delegate::LoadCompleted, delegate);
  d->RegisterHandler("setInspectedPageBounds",
                     &Delegate::SetInspectedPageBounds, delegate);
  d->RegisterHandler("inspectElementCompleted",
                     &Delegate::InspectElementCompleted, delegate);
  d->RegisterHandler("inspectedURLChanged", &Delegate::InspectedURLChanged,
                     delegate);
  d->RegisterHandlerWithCallback("setIsDocked", &Delegate::SetIsDocked,
                                 delegate);
  d->RegisterHandler("openInNewTab", &Delegate::OpenInNewTab, delegate);
  d->RegisterHandler("save", &Delegate::SaveToFile, delegate);
  d->RegisterHandler("append", &Delegate::AppendToFile, delegate);
  d->RegisterHandler("requestFileSystems", &Delegate::RequestFileSystems,
                     delegate);
  d->RegisterHandler("addFileSystem", &Delegate::AddFileSystem, delegate);
  d->RegisterHandler("removeFileSystem", &Delegate::RemoveFileSystem, delegate);