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

webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc

Index: webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc
diff --git a/webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc b/webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc
index cb0780ced04fce9634f4773683a23c07f491e2ff..15f7e15cd4c20ddd97f7282c2b36efa0f77ef2d7 100644
--- a/webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc
+++ b/webrtc/modules/audio_coding/neteq/test/neteq_ilbc_quality_test.cc
@@ -47,8 +47,7 @@ class NetEqIlbcQualityTest : public NetEqQualityTest {
       : NetEqQualityTest(FLAGS_frame_size_ms,
                          kInputSampleRateKhz,
                          kOutputSampleRateKhz,
-                         kDecoderILBC) {
-  }
+                         NetEqDecoder::kDecoderILBC) {}
 
   void SetUp() override {
     ASSERT_EQ(1, channels_) << "iLBC supports only mono audio.";