Fix incorrect AudioConverter setup for discrete channel layouts.

Fix incorrect AudioConverter setup for discrete channel layouts.

Discrete layouts will have a matching channel layout, but may have
any number of channels. We were incorrectly skipping the channel
mixer setup based only on layout.

BUG=473688
TEST=new unittest.

Committed: https://crrev.com/a46cde017f0f53a72eea9dd2f3c94e2b1336caee
Cr-Commit-Position: refs/heads/master@{#324351}

[DaleCurtis, 2015-04-08 20:43:16]

dalecurtis@chromium.org changed reviewers:
+ sandersd@chromium.org

[sandersd (OOO until July 31), 2015-04-08 23:23:21]

Well, I have no idea what a discrete channel layout is, but LGTM.

[DaleCurtis, 2015-04-09 00:08:18]

It was a feature (which isn't in use currently) for OSX which allowed users with
say 32 channel mixers and no known layout to hook into the mixer via WebAudio.
The WebAudio team is working on getting it enabled again.

[DaleCurtis, 2015-04-09 00:08:24]

The CQ bit was checked by dalecurtis@chromium.org

[commit-bot: I haz the power, 2015-04-09 00:08:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1070923002/1

[commit-bot: I haz the power, 2015-04-09 03:20:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-09 03:20:56]

Patchset 1 (id:??) landed as
https://crrev.com/a46cde017f0f53a72eea9dd2f3c94e2b1336caee
Cr-Commit-Position: refs/heads/master@{#324351}