Addressing @stesch13's timeout concerns

I raised the arbitrary-ish revocation timeout from 10ms to 500ms. I hope
to refine this value in the future with input from YOU, my users reading
this.

Please comment on this commit with any feedback you have.

@stesch13's concerns can be found at
https://github.com/eligrey/FileSaver.js/commit/485930a#commitcomment-8768047