* 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.cc
frontend_loaded_ = false;
  external_devtools_web_contents_ = nullptr;
  Observe(nullptr);
  Detach();
  embedder_message_dispatcher_.reset();
  for (const auto& pair : pending_requests_)
    delete pair.first;
  if (view_ && view_->GetDelegate())
    view_->GetDelegate()->DevToolsClosed();
}
bool InspectableWebContentsImpl::DidAddMessageToConsole(
    content::WebContents* source,
    int32_t level,
    const base::string16& message,
    int32_t line_no,
    const base::string16& source_id) {
  logging::LogMessage("CONSOLE", line_no, level).stream()
      << "\"" << message << "\", source: " << source_id << " (" << line_no
      << ")";
  return true;
}
bool InspectableWebContentsImpl::ShouldCreateWebContents(