WebmMuxer: consider both Audio and Video for muxer timestamps.

The WebmMuxer computes media timestamps with media timestamp origins
treated separately for audio and video. Thus if the Audio & Video
streams start with an offset in time, the webm media will be encoded
out of sync.

Fix this by using a single media timestamp origin, being the first
sample received regardless of whether it's an Audio or Video sample.

Additionally, the change adds UMAs:
Media.WebmMuxer.DidAdjustTimestamp.{StreamTypesRecorded}.{Domain}
where StreamTypesRecorder is in [AudioOnly, VideoOnly, AudioVideo] and
Domain [Muxer, Audio, Video] indicates if timestamps were corrected in
the session.

Fixed: 1304104
Change-Id: I575920413de379b9ad68361d8e61714fd496ce5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4134849
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Evan Liu <evliu@google.com>
Commit-Queue: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/main@{#1090275}