Bug 1749047 - Add BitrateMode to MediaRecorder.webidl. r=webidl,smaug

https://w3c.github.io/mediacapture-record/#bitratemode, we're not implementing
this on MediaRecorder, but are needed this for Web Codecs.

Differential Revision: https://phabricator.services.mozilla.com/D195702

dom/webidl/MediaRecorder.webidl

@@ -10,6 +10,9 @@
  * liability, trademark and document use rules apply.
  */
 
+
+enum BitrateMode { "constant", "variable" };
+
 enum RecordingState { "inactive", "recording", "paused" };
 
 [Exposed=Window]