[mediacapture-record] What are the default bitrate attribute values? (#169)

jan-ivar has just created a new issue for https://github.com/w3c/mediacapture-record:

== What are the default bitrate attribute values? ==
The [videoBitsPerSecond](https://w3c.github.io/mediacapture-record/MediaRecorder.html#dom-mediarecorder-videobitspersecond) read-only attribute says *"The value of the Video encoding target bit rate that was passed to the Platform (potentially truncated, rounded, etc), or the calculated one if the user has specified bitsPerSecond."*

This does not say what to return If neither `videoBitsPerSecond` nor `bitsPerSecond` were passed. Ditto for [audioBitsPerSecond](https://w3c.github.io/mediacapture-record/MediaRecorder.html#dom-mediarecorder-audiobitspersecond).

Possible solutions:
 1. return `0`
 2. Make `audioBitsPerSecond` and `videoBitsPerSecond` attributes nullable.


Please view or discuss this issue at https://github.com/w3c/mediacapture-record/issues/169 using your GitHub account