* 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
frontend_loaded_ = true;
  if (managed_devtools_web_contents_)
    view_->ShowDevTools();
  
  if (!can_dock_) {
    SetIsDocked(DispatchCallback(), false);
  } else {
    if (dock_state_.empty()) {
      const base::DictionaryValue* prefs =
          pref_service_->GetDictionary(kDevToolsPreferences);
      std::string current_dock_state;
      prefs->GetString("currentDockState", &current_dock_state);
      base::RemoveChars(current_dock_state, "\"", &dock_state_);
    }
    base::string16 javascript = base::UTF8ToUTF16(
        "Components.dockController.setDockSide(\"" + dock_state_ + "\");");
    GetDevToolsWebContents()->GetMainFrame()->ExecuteJavaScript(javascript);
  }
  if (view_->GetDelegate())
    view_->GetDelegate()->DevToolsOpened();
}
void InspectableWebContentsImpl::SetInspectedPageBounds(const gfx::Rect& rect) {
  DevToolsContentsResizingStrategy strategy(rect);
  if (contents_resizing_strategy_.Equals(strategy))
    return;
  contents_resizing_strategy_.CopyFrom(strategy);
  if (managed_devtools_web_contents_)
    view_->SetContentsResizingStrategy(contents_resizing_strategy_);
}
void InspectableWebContentsImpl::InspectElementCompleted() {}