GitLab

Replaces the use of MultiChannelResampler with AudioConverter in
FakeMediaSource to allow the cast_sender_app and cast_simulator to
re-mix any-number-of-channels audio into the 2-channel audio for Cast
sending.

Review URL: https://codereview.chromium.org/900913004

Cr-Commit-Position: refs/heads/master@{#314680}