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.cc b/modules/audio_coding/neteq/neteq_impl.cc
index 738e974..6836c7e 100644
--- a/modules/audio_coding/neteq/neteq_impl.cc
+++ b/modules/audio_coding/neteq/neteq_impl.cc
@@ -311,7 +311,7 @@
   return kNotImplemented;
 }
 
-int NetEqImpl::TargetDelayMs() {
+int NetEqImpl::TargetDelayMs() const {
   rtc::CritScope lock(&crit_sect_);
   RTC_DCHECK(delay_manager_.get());
   // The value from TargetLevel() is in number of packets, represented in Q8.