Fix getBitrate()

[avenal]

Since "mediaType" property seems deprecated as it was renamed to "kind" (https://developer.mozilla.org/en-US/docs/Web/API/RTCInboundRtpStreamStats), i would like to introduce small change in janus.js getBitrate function, so that it will return the correct values when called on iPhones and iPads

[januscla]

Thanks for your contribution, @avenal! Please make sure you sign our CLA, as it's a required step before we can merge this.

[lminiero]

Merging, thanks!