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

webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc

Index: webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc
diff --git a/webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc b/webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc
index 2a116163bfd7ac341873be260f3c40c2c56c8eab..09eb5614fe9d10bd58388c5ab5078ad076a3635a 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_external_decoder_unittest.cc
@@ -170,18 +170,20 @@ class NetEqExternalVsInternalDecoderTest : public NetEqExternalDecoderUnitTest,
   static const size_t kMaxBlockSize = 480;  // 10 ms @ 48 kHz.
 
   NetEqExternalVsInternalDecoderTest()
-      : NetEqExternalDecoderUnitTest(kDecoderPCM16Bswb32kHz,
+      : NetEqExternalDecoderUnitTest(NetEqDecoder::kDecoderPCM16Bswb32kHz,
                                      new MockExternalPcm16B),
-        sample_rate_hz_(CodecSampleRateHz(kDecoderPCM16Bswb32kHz)) {
+        sample_rate_hz_(
+            CodecSampleRateHz(NetEqDecoder::kDecoderPCM16Bswb32kHz)) {
     NetEq::Config config;
-    config.sample_rate_hz = CodecSampleRateHz(kDecoderPCM16Bswb32kHz);
+    config.sample_rate_hz =
+        CodecSampleRateHz(NetEqDecoder::kDecoderPCM16Bswb32kHz);
     neteq_internal_.reset(NetEq::Create(config));
   }
 
   void SetUp() override {
     ASSERT_EQ(NetEq::kOK,
-              neteq_internal_->RegisterPayloadType(kDecoderPCM16Bswb32kHz,
-                                                   kPayloadType));
+              neteq_internal_->RegisterPayloadType(
+                  NetEqDecoder::kDecoderPCM16Bswb32kHz, kPayloadType));
   }
 
   void GetAndVerifyOutput() override {
@@ -250,7 +252,7 @@ class LargeTimestampJumpTest : public NetEqExternalDecoderUnitTest,
   };
 
   LargeTimestampJumpTest()
-      : NetEqExternalDecoderUnitTest(kDecoderPCM16B,
+      : NetEqExternalDecoderUnitTest(NetEqDecoder::kDecoderPCM16B,
                                      new MockExternalPcm16B),
         test_state_(kInitialPhase) {
     EXPECT_CALL(*external_decoder(), HasDecodePlc())