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
Review URL: https://codereview.chromium.org/
1129383006
Cr-Commit-Position: refs/heads/master@{#329662}