Bug 1207119: [mp3] Properly read channels count. r=esawin

The channel count is stored on bit 6 & 7 of byte at offset 3.

dom/media/MP3Demuxer.cpp

@@ -688,7 +688,7 @@ FrameParser::FrameHeader::Private() const {
 
 uint8_t
 FrameParser::FrameHeader::RawChannelMode() const {
-  return 0xF & mRaw[frame_header::CHANNELMODE_MODEEXT_COPY_ORIG_EMPH] >> 4;
+  return 0x3 & mRaw[frame_header::CHANNELMODE_MODEEXT_COPY_ORIG_EMPH] >> 6;
 }
 
 int32_t