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Some audio input devices don't generate caputure data in a continuous basis, and
instead only generate when some event happens. This may be an indefinite amount
of time. However, |AudioSink::OnSetFormat| is only called whenever new samples
come in. For pepper plugins, this means that |MediaStreamAudioTrack::Configure|
can be blocked indefintely on these devices. To work around this, get the
format before attaching to the audio track.

BUG=403138

Review URL: https://codereview.chromium.org/857093002

Cr-Commit-Position: refs/heads/master@{#313458}