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

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

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 29 matching lines @@
     RegisterFlagValidator(&FLAGS_frame_size_ms, &ValidateFrameSize);
 
 }  // namespace
 
 class NetEqPcmuQualityTest : public NetEqQualityTest {
  protected:
   NetEqPcmuQualityTest()
       : NetEqQualityTest(FLAGS_frame_size_ms,
                          kInputSampleRateKhz,
                          kOutputSampleRateKhz,
-                         kDecoderPCMu) {
-  }
+                         NetEqDecoder::kDecoderPCMu) {}
 
   void SetUp() override {
     ASSERT_EQ(1, channels_) << "PCMu supports only mono audio.";
     AudioEncoderPcmU::Config config;
     config.frame_size_ms = FLAGS_frame_size_ms;
     encoder_.reset(new AudioEncoderPcmU(config));
     NetEqQualityTest::SetUp();
   }
 
   int EncodeBlock(int16_t* in_data,
@@ skipping 15 matching lines @@
  private:
   rtc::scoped_ptr<AudioEncoderPcmU> encoder_;
 };
 
 TEST_F(NetEqPcmuQualityTest, Test) {
   Simulate();
 }
 
 }  // namespace test
 }  // namespace webrtc