Add chrome://flags entry for --try-supported-channel-layouts

Add chrome://flags entry for --try-supported-channel-layouts

Allows stereo channel expansion for many users. We unfortunately
can't turn it on for everyone due to bugs in third party drive
implementations :( -- see http://crbug.com/259165

Many users have been using this flag diligently for a while now,
but every Chrome update wipes the flag, so make it a chrome://flag
to improve usability for these users.

BUG=479057
TEST=none

Committed: https://crrev.com/8e5e2f47b4fc3efebac06b8c1ff0a5e049a95be2
Cr-Commit-Position: refs/heads/master@{#329662}

[DaleCurtis, 2015-05-12 18:15:23]

Patchset #2 (id:20001) has been deleted

[DaleCurtis, 2015-05-12 18:15:31]

Patchset #1 (id:1) has been deleted

[DaleCurtis, 2015-05-12 18:18:35]

Patchset #1 (id:40001) has been deleted

[DaleCurtis, 2015-05-12 18:19:22]

dalecurtis@chromium.org changed reviewers:
+ asvitkine@chromium.org, henrika@chromium.org

[Alexei Svitkine (slow), 2015-05-12 18:20:44]

lgtm

[henrika (OOO until Aug 14), 2015-05-13 06:50:19]

Thanks! LGTM

[DaleCurtis, 2015-05-13 16:11:35]

The CQ bit was checked by dalecurtis@chromium.org

[commit-bot: I haz the power, 2015-05-13 16:11:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1129383006/60001

[commit-bot: I haz the power, 2015-05-13 17:16:59]

Committed patchset #1 (id:60001)

[commit-bot: I haz the power, 2015-05-13 17:17:44]

Patchset 1 (id:??) landed as
https://crrev.com/8e5e2f47b4fc3efebac06b8c1ff0a5e049a95be2
Cr-Commit-Position: refs/heads/master@{#329662}