* 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
public:
  ResponseWriter(base::WeakPtr<InspectableWebContentsImpl> bindings,
                 int stream_id);
  ~ResponseWriter() override;
  
  int Initialize(const net::CompletionCallback& callback) override;
  int Write(net::IOBuffer* buffer,
            int num_bytes,
            const net::CompletionCallback& callback) override;
  int Finish(int net_error, const net::CompletionCallback& callback) override;
 private:
  base::WeakPtr<InspectableWebContentsImpl> bindings_;
  int stream_id_;
  DISALLOW_COPY_AND_ASSIGN(ResponseWriter);
};
ResponseWriter::ResponseWriter(