* 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
void InspectableWebContentsImpl::InspectedURLChanged(const std::string& url) {
  if (managed_devtools_web_contents_)
    view_->SetTitle(
        base::UTF8ToUTF16(base::StringPrintf(kTitleFormat, url.c_str())));
}
void InspectableWebContentsImpl::LoadNetworkResource(
    const DispatchCallback& callback,
    const std::string& url,
    const std::string& headers,
    int stream_id) {
  GURL gurl(url);
  if (!gurl.is_valid()) {
    base::DictionaryValue response;
    response.SetInteger("statusCode", 404);
    callback.Run(&response);
    return;
  }
  auto* browser_context = static_cast<BrowserContext*>(
      GetDevToolsWebContents()->GetBrowserContext());
  net::URLFetcher* fetcher =
      (net::URLFetcher::Create(gurl, net::URLFetcher::GET, this)).release();
  pending_requests_[fetcher] = callback;
  fetcher->SetRequestContext(browser_context->url_request_context_getter());
  fetcher->SetExtraRequestHeaders(headers);
  fetcher->SaveResponseWithWriter(
      std::unique_ptr<net::URLFetcherResponseWriter>(
          new ResponseWriter(weak_factory_.GetWeakPtr(), stream_id)));
  fetcher->Start();
}
void InspectableWebContentsImpl::SetIsDocked(const DispatchCallback& callback,