Surround Sound handling by folding 5 channels down to stereo.

      BUG=16026
TEST=play a movie trailer with 5.1 audio.  It should sound roughly the same as quicktime.

Review URL: http://codereview.chromium.org/155894

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@21613 0039d316-1c4b-4281-b951-d872f2087c98