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* Electron version: 1.6.1
* Operating system: macOS

### Expected behavior

navigator.geolocation.getCurrentPosition runs in background

### Actual behavior

the entire UI is frozen

### How to reproduce

```js
    navigator.geolocation.getCurrentPosition((pos) => {
      console.log('POS: ' + pos);
    }, (err) => {
      console.log('Geolocation error: ', err);
    }, {enableHighAccuracy: false});
```
atom/browser/api/atom_api_web_contents.cc
atom::api::WebContents::Type val) {
    using Type = atom::api::WebContents::Type;
    std::string type = "";
    switch (val) {
      case Type::BACKGROUND_PAGE: type = "backgroundPage"; break;
      case Type::BROWSER_WINDOW: type = "window"; break;
      case Type::REMOTE: type = "remote"; break;
      case Type::WEB_VIEW: type = "webview"; break;
      case Type::OFF_SCREEN: type = "offscreen"; break;
      default: break;
    }
    return mate::ConvertToV8(isolate, type);
  }
  static bool FromV8(v8::Isolate* isolate, v8::Local<v8::Value> val,