Reland: Implement NetEq's CurrentDelay function

This was not implemented before. It returns the current total delay
(packet buffer and sync buffer) of NetEq. This is the same information
that was already available in
NetEqNetworkStatistics::current_buffer_size_ms, that can be obtained
through NetEq::NetworkStatistics(). But, since the current delay is a
key metric of NetEq, it is convenient to have it available in a
simpler way.

This is a re-landing of r9359,
https://webrtc-codereview.appspot.com/51149004, which was reverted in
r9360. The refactoring made in r9669 facilitated the relanding.

TBR=minyue@webrtc.org

Review URL: https://codereview.webrtc.org/1313873003

Cr-Commit-Position: refs/heads/master@{#9801}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 182d8b8..6a45db6 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -136,7 +136,7 @@
 
   int TargetDelay() override;
 
-  int CurrentDelay() override;
+  int CurrentDelayMs() const override;
 
   // Sets the playout mode to |mode|.
   // Deprecated.