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

This operation was relatively simple, since no one was doing anything
fishy with this enum. A large number of lines had to be changed
because the enum values now live in their own namespace, but this is
arguably worth it since it is now much clearer what sort of constant
they are.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1424083002

Cr-Commit-Position: refs/heads/master@{#10449}
diff --git a/webrtc/modules/audio_coding/neteq/timestamp_scaler.cc b/webrtc/modules/audio_coding/neteq/timestamp_scaler.cc
index 0b0c737..eb69ac7 100644
--- a/webrtc/modules/audio_coding/neteq/timestamp_scaler.cc
+++ b/webrtc/modules/audio_coding/neteq/timestamp_scaler.cc
@@ -44,15 +44,15 @@
     return external_timestamp;
   }
   switch (info->codec_type) {
-    case kDecoderG722:
-    case kDecoderG722_2ch: {
+    case NetEqDecoder::kDecoderG722:
+    case NetEqDecoder::kDecoderG722_2ch: {
       // Use timestamp scaling with factor 2 (two output samples per RTP
       // timestamp).
       numerator_ = 2;
       denominator_ = 1;
       break;
     }
-    case kDecoderCNGswb48kHz: {
+    case NetEqDecoder::kDecoderCNGswb48kHz: {
       // Use timestamp scaling with factor 2/3 (32 kHz sample rate, but RTP
       // timestamps run on 48 kHz).
       // TODO(tlegrand): Remove scaling for kDecoderCNGswb48kHz once ACM has
@@ -61,10 +61,10 @@
       denominator_ = 3;
       break;
     }
-    case kDecoderAVT:
-    case kDecoderCNGnb:
-    case kDecoderCNGwb:
-    case kDecoderCNGswb32kHz: {
+    case NetEqDecoder::kDecoderAVT:
+    case NetEqDecoder::kDecoderCNGnb:
+    case NetEqDecoder::kDecoderCNGwb:
+    case NetEqDecoder::kDecoderCNGswb32kHz: {
       // Do not change the timestamp scaling settings for DTMF or CNG.
       break;
     }