* 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
content::WebContents* devtools) {
  if (!managed_devtools_web_contents_)
    external_devtools_web_contents_ = devtools;
}
void InspectableWebContentsImpl::ShowDevTools() {
  if (embedder_message_dispatcher_) {
    if (managed_devtools_web_contents_)
      view_->ShowDevTools();
    return;
  }
  
  
  embedder_message_dispatcher_.reset(
      DevToolsEmbedderMessageDispatcher::CreateForDevToolsFrontend(this));
  if (!external_devtools_web_contents_) {  
    managed_devtools_web_contents_.reset(
        content::WebContents::Create(content::WebContents::CreateParams(
            web_contents_->GetBrowserContext())));
    managed_devtools_web_contents_->SetDelegate(this);
  }
  Observe(GetDevToolsWebContents());
  AttachTo(content::DevToolsAgentHost::GetOrCreateFor(web_contents_.get()));
  GetDevToolsWebContents()->GetController().LoadURL(
      GetDevToolsURL(can_dock_), content::Referrer(),
      ui::PAGE_TRANSITION_AUTO_TOPLEVEL, std::string());
}
void InspectableWebContentsImpl::CloseDevTools() {