* 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
std::unique_ptr<base::DictionaryValue> bounds_dict(new base::DictionaryValue);
  RectToDictionary(gfx::Rect(0, 0, 800, 600), bounds_dict.get());
  registry->RegisterDictionaryPref(kDevToolsBoundsPref, std::move(bounds_dict));
  registry->RegisterDoublePref(kDevToolsZoomPref, 0.);
  registry->RegisterDictionaryPref(kDevToolsPreferences);
}
InspectableWebContentsImpl::InspectableWebContentsImpl(
    content::WebContents* web_contents)
    : frontend_loaded_(false),
      can_dock_(true),
      delegate_(nullptr),
      web_contents_(web_contents),
      weak_factory_(this) {
  auto* context =
      static_cast<BrowserContext*>(web_contents_->GetBrowserContext());
  pref_service_ = context->prefs();
  auto* bounds_dict = pref_service_->GetDictionary(kDevToolsBoundsPref);
  if (bounds_dict) {
    DictionaryToRect(*bounds_dict, &devtools_bounds_);
    
    if (devtools_bounds_.height() < 100 || devtools_bounds_.width() < 100) {
      devtools_bounds_.set_height(600);
      devtools_bounds_.set_width(800);
    }
    if (!IsPointInScreen(devtools_bounds_.origin())) {
      gfx::Rect display;