Support webkitSlice if slice is not available

I found this issue when I was testing a functionality that uses stream parsing with `capybara-webkit`.

If `file.slice` is not present, use `file.webkitSlice`.

See this [reference](https://github.com/blueimp/jQuery-File-Upload/issues/1704).

For some reason, `slice = file.slice || file.webkitSlice` does not work, so that's why the implementation looks awkward.