* 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::RenderWidgetHost* render_widget_host) {
#if defined(TOOLKIT_VIEWS)
  if (view_->GetDelegate())
    view_->GetDelegate()->DevToolsFocused();
#endif
}
void InspectableWebContentsImpl::ReadyToCommitNavigation(
    content::NavigationHandle* navigation_handle) {
  if (navigation_handle->IsInMainFrame()) {
    if (navigation_handle->GetRenderFrameHost() ==
            GetDevToolsWebContents()->GetMainFrame() &&
        frontend_host_) {
      return;
    }
    frontend_host_.reset(content::DevToolsFrontendHost::Create(
        web_contents()->GetMainFrame(),
        base::Bind(
            &InspectableWebContentsImpl::HandleMessageFromDevToolsFrontend,
            base::Unretained(this))));
    return;
  }
}
void InspectableWebContentsImpl::DidFinishNavigation(