* 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
const net::URLFetcher* source) {
  DCHECK(source);
  auto it = pending_requests_.find(source);
  DCHECK(it != pending_requests_.end());
  base::DictionaryValue response;
  net::HttpResponseHeaders* rh = source->GetResponseHeaders();
  response.SetInteger("statusCode", rh ? rh->response_code() : 200);
  {
    auto headers = std::make_unique<base::DictionaryValue>();
    size_t iterator = 0;
    std::string name;
    std::string value;
    while (rh && rh->EnumerateHeaderLines(&iterator, &name, &value))
      headers->SetString(name, value);
    response.Set("headers", std::move(headers));
  }
  it->second.Run(&response);
  pending_requests_.erase(it);
  delete source;
}
void InspectableWebContentsImpl::SendMessageAck(int request_id,
                                                const base::Value* arg) {
  base::Value id_value(request_id);
  CallClientFunction("DevToolsAPI.embedderMessageAck", &id_value, arg, nullptr);
}
}