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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
mate::Arguments* args) {
  base::FilePath file;
  std::vector<base::FilePath> files;
  if (!item.Get("files", &files) && item.Get("file", &file)) {
    files.push_back(file);
  }
  mate::Handle<NativeImage> icon;
  if (!item.Get("icon", &icon) && !file.empty()) {
    
  }
  
  if (icon.IsEmpty()) {
    args->ThrowError("Must specify 'icon' option");
    return;
  }
#if defined(OS_MACOSX)
  
  if (icon->image().IsEmpty()) {
    args->ThrowError("Must specify non-empty 'icon' option");
    return;
  }
#endif
  
  if (!files.empty()) {
    base::MessageLoop::ScopedNestableTaskAllower allow(
        base::MessageLoop::current());
    DragFileItems(files, icon->image(), web_contents()->GetNativeView());
  } else {