Resolves crash in media::ChannelMixer::Transform.

New version ensures that input and output bus to/from the channel mixer has the same size.

BUG=250947
TEST=Verified that unified audio clients crashed when input device used mono and we did resampling (e.g. 16->32). For this case, different audio bus sizes were used by the channel mixer. Using the fix, the channel mixer now always uses input and output buses of the same size (320 and 320 in the case above)

Review URL: https://chromiumcodereview.appspot.com/17377004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@207053 0039d316-1c4b-4281-b951-d872f2087c98