[mediacapture-record] Provide Surface API for reading back the configured encoding bitrate(s)

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

== Provide Surface API for reading back the configured encoding 
bitrate(s) ==
Section [MediaRecorder 
Properties](https://rawgit.com/w3c/mediacapture-record/master/MediaRecorder.html#properties)
 says:
> [TO DO: Provide API surface to set and read these. When that is 
done, this section will be removed.]

this is the issue for that TODO, for the bitrate options concretely.

(It's basically a follow up of @mreavy "improve bitrate control in 
MediaRecorder" #20 - discussed with @tdaede).

Please view or discuss this issue at 
https://github.com/w3c/mediacapture-record/issues/43 using your GitHub
 account