Revert of Remove the last piece of deprecated synchronous IO code.

media/audio/mac/audio_manager_mac.cc

Index: media/audio/mac/audio_manager_mac.cc
diff --git a/media/audio/mac/audio_manager_mac.cc b/media/audio/mac/audio_manager_mac.cc
index 336a8c4994db7eb71da84ff22f837d125cb09318..eabce32381920bd541e983c607b0e7ab6a4023e1 100644
--- a/media/audio/mac/audio_manager_mac.cc
+++ b/media/audio/mac/audio_manager_mac.cc
@@ -674,9 +674,20 @@
       channel_layout = CHANNEL_LAYOUT_DISCRETE;
   }
 
+  const int input_channels =
+      has_valid_input_params ? input_params.input_channels() : 0;
+  if (input_channels > 0) {
+    // TODO(xians): given the limitations of the AudioOutputStream
+    // back-ends used with synchronized I/O, we hard-code to stereo.
+    // Specifically, this is a limitation of AudioSynchronizedStream which
+    // can be removed as part of the work to consolidate these back-ends.
+    channel_layout = CHANNEL_LAYOUT_STEREO;
+  }
+
   return AudioParameters(
       AudioParameters::AUDIO_PCM_LOW_LATENCY, channel_layout, output_channels,
-      hardware_sample_rate, 16, buffer_size, AudioParameters::NO_EFFECTS);
+      input_channels, hardware_sample_rate, 16, buffer_size,
+      AudioParameters::NO_EFFECTS);
 }
 
 void AudioManagerMac::InitializeOnAudioThread() {