* 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
return web_contents_.get();
}
content::WebContents* InspectableWebContentsImpl::GetDevToolsWebContents()
    const {
  if (external_devtools_web_contents_)
    return external_devtools_web_contents_;
  else
    return managed_devtools_web_contents_.get();
}
void InspectableWebContentsImpl::InspectElement(int x, int y) {
  if (agent_host_.get())
    agent_host_->InspectElement(web_contents_->GetMainFrame(), x, y);
}
void InspectableWebContentsImpl::SetDelegate(
    InspectableWebContentsDelegate* delegate) {
  delegate_ = delegate;
}
InspectableWebContentsDelegate* InspectableWebContentsImpl::GetDelegate()
    const {
  return delegate_;
}
void InspectableWebContentsImpl::SetDockState(const std::string& state) {
  if (state == "detach") {
    can_dock_ = false;
  } else {
    can_dock_ = true;
    dock_state_ = state;
  }
}
void InspectableWebContentsImpl::SetDevToolsWebContents(