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This change fixes a problem where the WebmMuxer will
produce duplicate timestamps leading to audio gaps or
jerky video in WebM files, provided both audio and
video is recorded, and timestamps of all incoming
frames are not monotonically increasing. This
resulted in the WebmMuxer rewriting apparently older
frames' timestamps as the newest one encountered.

With this change we now normally don't change the incoming
timestamps, but always output a sorted sequence of frames
to the underlying Matroska muxer. Timestamps are
exceptionally changed when it's found that each stream's
timestamps aren't monotonically increasing.

Additionally, the webm_muxer got it's unit tests updated.

Bug: 873963
Change-Id: I6b81ef32f0f7f1034a56da3bca07186221fefaea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033253
Commit-Queue: Markus Handell <handellm@google.com>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738738}