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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
const SkBitmap& bitmap,
                       content::ReadbackResponse response) {
  callback.Run(gfx::Image::CreateFrom1xBitmap(bitmap));
}
void SetBackgroundColor(content::WebContents* web_contents) {
  const auto view = web_contents->GetRenderWidgetHostView();
  if (view) {
    WebContentsPreferences* web_preferences =
        WebContentsPreferences::FromWebContents(web_contents);
    std::string color_name;
    if (web_preferences->web_preferences()->GetString(options::kBackgroundColor,
                                                      &color_name)) {
      view->SetBackgroundColor(ParseHexColor(color_name));
    } else {
      view->SetBackgroundColor(SK_ColorTRANSPARENT);
    }
  }
}
}  
WebContents::WebContents(v8::Isolate* isolate,