Expose the internal MatrixBuilder class as ChannelMixingMatrix.

media/base/channel_mixer_unittest.cc

Index: media/base/channel_mixer_unittest.cc
diff --git a/media/base/channel_mixer_unittest.cc b/media/base/channel_mixer_unittest.cc
index ac228e661dd39bb5869b4e98ddafa624807cdc4a..e07d95110ee362385f10da13be7ae2823eb234a0 100644
--- a/media/base/channel_mixer_unittest.cc
+++ b/media/base/channel_mixer_unittest.cc
@@ -24,6 +24,7 @@ TEST(ChannelMixerTest, ConstructAllPossibleLayouts) {
        input_layout <= CHANNEL_LAYOUT_MAX;
        input_layout = static_cast<ChannelLayout>(input_layout + 1)) {
     for (ChannelLayout output_layout = CHANNEL_LAYOUT_MONO;
+         // TODO(wtc): why do we only test up to CHANNEL_LAYOUT_STEREO_DOWNMIX?

[DaleCurtis, 2014/10/23 16:49:05]

Hmm, I don't remember, may be a bug. What happens if you change it to MAX? Is
there any git blame info on that line?

[wtc, 2014/10/23 18:39:45]

If I change it to MAX, this CHECK will fail:

  // Stereo down mix should never be the output layout.
  CHECK_NE(output_layout, CHANNEL_LAYOUT_STEREO_DOWNMIX);

I can fix the CHECK failure by skipping the test if output_layout ==
CHANNEL_LAYOUT_STEREO_DOWNMIX on line 35.
Yes. You added it in the initial revision of this file:

https://chromiumcodereview.appspot.com/11150034

[DaleCurtis, 2014/10/23 19:23:50]

Haha, whoops. Thanks for fixing :)

          output_layout < CHANNEL_LAYOUT_STEREO_DOWNMIX;
          output_layout = static_cast<ChannelLayout>(output_layout + 1)) {
       // DISCRETE can't be tested here based on the current approach.