NetEq: Ask AudioDecoder for sample rate instead of passing it as an argument

BUG=webrtc:5801
NOTRY=true

Review-Url: https://codereview.webrtc.org/2027993002
Cr-Commit-Position: refs/heads/master@{#13162}

webrtc/modules/audio_coding/neteq/decoder_database_unittest.cc

@@ -141,7 +141,7 @@ TEST(DecoderDatabase, ExternalDecoder) {
   // Load into database.
   EXPECT_EQ(DecoderDatabase::kOK,
             db.InsertExternal(kPayloadType, NetEqDecoder::kDecoderPCMu,
-                              kCodecName, 8000, &decoder));
+                              kCodecName, &decoder));
   EXPECT_EQ(1, db.Size());
   // Get decoder and make sure we get the external one.
   EXPECT_EQ(&decoder, db.GetDecoder(kPayloadType));