Add support for channel transforms.

media/base/channel_layout.cc

Index: media/base/channel_layout.cc
diff --git a/media/base/channel_layout.cc b/media/base/channel_layout.cc
index b8b55001e3191f5269cbdd405540c2bd5951b7b5..a95ac002f87447cd011161ef12d485c23653877e 100644
--- a/media/base/channel_layout.cc
+++ b/media/base/channel_layout.cc
@@ -86,7 +86,7 @@ static const int kChannelOrderings[CHANNEL_LAYOUT_MAX][CHANNELS_MAX] = {
   {  0  , 1  , 2  , 3   , 4  , 5  , 6     , 7     , -1 , -1 , -1 , -1  , -1 },
 
   // CHANNEL_LAYOUT_STEREO_DOWNMIX
-  {  -1 , -1 , -1 , -1  , -1 , -1 , -1    , -1    , -1 , -1 , -1 , 0   , 1  },
+  {  0  , 1  , -1 , -1  , -1 , -1 , -1    , -1    , -1 , -1 , -1 , -1  , -1 },

[scherkus (not reviewing), 2012/10/18 01:30:39]

is this a bug fix that didn't happen when DOWNMIX was added?

can we document the difference between FL/FR and StL/StR on the Channels enum?

[DaleCurtis, 2012/10/18 01:56:44]

I'm not sure what those channels actually mean and no one is using them. Do you
have some historical context on this? Internally in FFmpeg's rematrixer they
just set output = STEREO when STEREO_DOWNMIX is encountered.

Barring any other information, I'll probably just delete those last two.

[scherkus (not reviewing), 2012/10/18 02:24:03]

No clue. You'll have to dig through commit logs + code reviews.

[DaleCurtis, 2012/10/18 05:22:04]

Removed. History shows they were just added because FFmpeg has them.  I couldn't
find any UMA stats using CHANNELS_MAX either, so looks okay to delete.

 
   // FL | FR | FC | LFE | BL | BR | FLofC | FRofC | BC | SL | SR | StL | StR
   };