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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/common_web_contents_delegate.cc
auto file_system_paths = GetAddedFileSystemPaths(GetDevToolsWebContents());
  if (file_system_paths.empty()) {
    base::ListValue empty_file_system_value;
    web_contents_->CallClientFunction("DevToolsAPI.fileSystemsLoaded",
                                      &empty_file_system_value,
                                      nullptr, nullptr);
    return;
  }
  std::vector<FileSystem> file_systems;
  for (auto file_system_path : file_system_paths) {
    base::FilePath path = base::FilePath::FromUTF8Unsafe(file_system_path);
    std::string file_system_id = RegisterFileSystem(GetDevToolsWebContents(),
                                                    path);
    FileSystem file_system = CreateFileSystemStruct(GetDevToolsWebContents(),
                                                    file_system_id,
                                                    file_system_path);
    file_systems.push_back(file_system);
  }
  base::ListValue file_system_value;
  for (const auto& file_system : file_systems)
    file_system_value.Append(CreateFileSystemValue(file_system));
  web_contents_->CallClientFunction("DevToolsAPI.fileSystemsLoaded",
                                    &file_system_value, nullptr, nullptr);
}
void CommonWebContentsDelegate::DevToolsAddFileSystem(