PSA: correct MediaRecorder.start() behaviour to buffer-up-to-forever

[Niklas Enbom]

Spec: https://w3c.github.io/mediacapture-record/MediaRecorder.html#dom-mediarecorder-start

Summary: MediaRecorder start() method accepts an optional |timeslice| argument. If and when |timeslice| is not present should be understood as 'infinite', whereas the current implementation erroneously interprets it as 0+.

Interoperability and Compatibility Risk: Interop is the motivation, Firefox already has this behaviour and users expect it.

This was pointed out in https://crbug.com/567859. 

There's already a CL addressing this bug in https://crrev.com/2444153002.