Make an enum class out of NetEqDecoder, and hide the neteq_decoders_ table

webrtc/modules/audio_coding/neteq/decoder_database.h

Index: webrtc/modules/audio_coding/neteq/decoder_database.h
diff --git a/webrtc/modules/audio_coding/neteq/decoder_database.h b/webrtc/modules/audio_coding/neteq/decoder_database.h
index 8ce0b696e79389c205e1c81b1bb53477f2115802..ea70997c14e9d321c73fcfb942fef686aae7cf04 100644
--- a/webrtc/modules/audio_coding/neteq/decoder_database.h
+++ b/webrtc/modules/audio_coding/neteq/decoder_database.h
@@ -37,11 +37,10 @@ class DecoderDatabase {
   struct DecoderInfo {
     // Constructors.
     DecoderInfo()
-        : codec_type(kDecoderArbitrary),
+        : codec_type(NetEqDecoder::kDecoderArbitrary),
           fs_hz(8000),
           decoder(NULL),
-          external(false) {
-    }
+          external(false) {}
     DecoderInfo(NetEqDecoder ct, int fs, AudioDecoder* dec, bool ext)
         : codec_type(ct),
           fs_hz(fs),