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..e818928f80eee8508885661ba07cb12b56fe5b81 100644
--- a/media/base/channel_mixer_unittest.cc
+++ b/media/base/channel_mixer_unittest.cc
@@ -24,14 +24,16 @@ TEST(ChannelMixerTest, ConstructAllPossibleLayouts) {
        input_layout <= CHANNEL_LAYOUT_MAX;
        input_layout = static_cast<ChannelLayout>(input_layout + 1)) {
     for (ChannelLayout output_layout = CHANNEL_LAYOUT_MONO;
-         output_layout < CHANNEL_LAYOUT_STEREO_DOWNMIX;
+         output_layout <= CHANNEL_LAYOUT_MAX;
          output_layout = static_cast<ChannelLayout>(output_layout + 1)) {
       // DISCRETE can't be tested here based on the current approach.
       // CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC is not mixable.
+      // Stereo down mix should never be the output layout.
       if (input_layout == CHANNEL_LAYOUT_DISCRETE ||
           input_layout == CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC ||
           output_layout == CHANNEL_LAYOUT_DISCRETE ||
-          output_layout == CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC) {
+          output_layout == CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC ||
+          output_layout == CHANNEL_LAYOUT_STEREO_DOWNMIX) {
         continue;
       }