* 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
if (GetDevToolsWebContents()) {
    frontend_loaded_ = false;
    if (managed_devtools_web_contents_) {
      view_->CloseDevTools();
      managed_devtools_web_contents_.reset();
    }
    embedder_message_dispatcher_.reset();
    web_contents_->Focus();
  }
}
bool InspectableWebContentsImpl::IsDevToolsViewShowing() {
  return managed_devtools_web_contents_ && view_->IsDevToolsViewShowing();
}
void InspectableWebContentsImpl::AttachTo(
    scoped_refptr<content::DevToolsAgentHost> host) {
  if (agent_host_.get())
    Detach();
  agent_host_ = std::move(host);
  
  agent_host_->ForceAttachClient(this);
}
void InspectableWebContentsImpl::Detach() {