Add new UMA metric for NetEq target buffer delay

The UMA metric will log the same information that goes into the
googPreferredJitterBufferMs stat.

Bug: webrtc:8488
Change-Id: I4e4e1e362dd42377105d52d2c4cd49c1ecb1a90d
Reviewed-on: https://webrtc-review.googlesource.com/26740
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20923}
diff --git a/modules/audio_coding/neteq/neteq_impl.h b/modules/audio_coding/neteq/neteq_impl.h
index 8dca6a5..a922bcd 100644
--- a/modules/audio_coding/neteq/neteq_impl.h
+++ b/modules/audio_coding/neteq/neteq_impl.h
@@ -161,7 +161,7 @@
 
   int SetTargetDelay() override;
 
-  int TargetDelayMs() override;
+  int TargetDelayMs() const override;
 
   int CurrentDelayMs() const override;