Bug 1504944 - Only re-order channels for opus mapping 1. r=padenot

For channel mapping 0 (which is always mono or sterero), the mapping is identical to the SMPTE one, so can copy the mapping as-is.

Differential Revision: https://phabricator.services.mozilla.com/D11000

--HG--
extra : moz-landing-system : lando

dom/media/platforms/agnostic/OpusDecoder.cpp

@@ -154,13 +154,13 @@ OpusDataDecoder::DecodeHeader(const unsigned char* aData, size_t aLength)
 
     AutoTArray<uint8_t, 8> map;
     map.SetLength(channels);
-    if (vorbisLayout.MappingTable(smpteLayout, &map)) {
+    if (mOpusParser->mChannelMapping == 1 &&
+        vorbisLayout.MappingTable(smpteLayout, &map)) {
       for (int i = 0; i < channels; i++) {
         mMappingTable[i] = mOpusParser->mMappingTable[map[i]];
       }
     } else {
-      // Should never get here as vorbis layout is always convertible to SMPTE
-      // default layout.
+      // Use Opus set channel mapping and return channels as-is.
       PodCopy(mMappingTable.Elements(), mOpusParser->mMappingTable, channels);
     }
   } else {