Added a progress function to util.downloadFile (#348)
Added a progress function to util.downloadFile (#348)

- When set, the function is invoked for every chunk received.
- If we're unable to determine the total file size, the function
  is invoked with totalSize as -1.
GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap,GoogleChromeLabs/bubblewrap