* 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
dict->SetInteger("x", bounds.x());
  dict->SetInteger("y", bounds.y());
  dict->SetInteger("width", bounds.width());
  dict->SetInteger("height", bounds.height());
}
void DictionaryToRect(const base::DictionaryValue& dict, gfx::Rect* bounds) {
  int x = 0, y = 0, width = 800, height = 600;
  dict.GetInteger("x", &x);
  dict.GetInteger("y", &y);
  dict.GetInteger("width", &width);
  dict.GetInteger("height", &height);
  *bounds = gfx::Rect(x, y, width, height);
}
bool IsPointInRect(const gfx::Point& point, const gfx::Rect& rect) {
  return point.x() > rect.x() && point.x() < (rect.width() + rect.x()) &&
         point.y() > rect.y() && point.y() < (rect.height() + rect.y());
}
bool IsPointInScreen(const gfx::Point& point) {
  for (const auto& display : display::Screen::GetScreen()->GetAllDisplays()) {
    if (IsPointInRect(point, display.bounds()))
      return true;
  }
  return false;
}
void SetZoomLevelForWebContents(content::WebContents* web_contents,