[cast/test] Use AudioConverter in FakeMediaSource to play back 6-channel WEBM's.

[cast/test] Use AudioConverter in FakeMediaSource to play back 6-channel WEBM's.

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.

Committed: https://crrev.com/267652edbd64fd42e71ed4f9557e0d807518c018
Cr-Commit-Position: refs/heads/master@{#314680}

[miu, 2015-02-04 23:01:37]

miu@chromium.org changed reviewers:
+ hclam@chromium.org

[miu, 2015-02-04 23:01:38]

hclam: PTAL.

[Alpha Left Google, 2015-02-04 23:07:24]

The CQ bit was checked by hclam@chromium.org

[Alpha Left Google, 2015-02-04 23:07:24]

lgtm

[commit-bot: I haz the power, 2015-02-04 23:08:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/900913004/1

[commit-bot: I haz the power, 2015-02-04 23:49:20]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-04 23:50:39]

Patchset 1 (id:??) landed as
https://crrev.com/267652edbd64fd42e71ed4f9557e0d807518c018
Cr-Commit-Position: refs/heads/master@{#314680}