Replacing NetEq discard rate with secondary discarded rate.

NetEq network statistics contains discard rate but has not been used and even not been implemented until recently.

According to w3c.github.io/webrtc-stats/#dom-rtcreceivedrtpstreamstats-packetsdiscarded,
this statistics needs to be replaced with an accumulative stats. Such work will be carried out separately.

Meanwhile, we need to add a rate to reflect rate of discarded redundant packets. See webrtc:8025.

In this CL, we replace the existing discard rate with secondary discarded rate, so as to
1. fulfill the requests on webrtc:8025
2. get ready to implement an accumulative statistics for discarded packets.

BUG: webrtc:7903,webrtc:8025
Change-Id: Idbf143a105db76ca15f0af54848e1448f2a810ec
Reviewed-on: https://chromium-review.googlesource.com/582863
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Minyue Li <minyue@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19495}
diff --git a/webrtc/modules/audio_coding/test/APITest.cc b/webrtc/modules/audio_coding/test/APITest.cc
index d18cdef..7e67e3f 100644
--- a/webrtc/modules/audio_coding/test/APITest.cc
+++ b/webrtc/modules/audio_coding/test/APITest.cc
@@ -752,8 +752,6 @@
             networkStat.jitterPeaksFound);
     fprintf(stdout, "packet-size rate........ %d\n",
             networkStat.currentPacketLossRate);
-    fprintf(stdout, "discard rate............ %d\n",
-            networkStat.currentDiscardRate);
     fprintf(stdout, "expand rate............. %d\n",
             networkStat.currentExpandRate);
     fprintf(stdout, "speech expand rate...... %d\n",
@@ -764,6 +762,8 @@
             networkStat.currentAccelerateRate);
     fprintf(stdout, "Secondary decoded rate.. %d\n",
             networkStat.currentSecondaryDecodedRate);
+    fprintf(stdout, "Secondary discarded rate.%d\n",
+            networkStat.currentSecondaryDiscardedRate);
     fprintf(stdout, "Clock-drift............. %d\n", networkStat.clockDriftPPM);
     fprintf(stdout, "Mean waiting time....... %d\n",
             networkStat.meanWaitingTimeMs);