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}

modules/audio_coding/neteq/include/neteq.h

@@ -208,7 +208,7 @@ class NetEq {
 
   // Returns the current target delay in ms. This includes any extra delay
   // requested through SetMinimumDelay.
-  virtual int TargetDelayMs() = 0;
+  virtual int TargetDelayMs() const = 0;
 
   // Returns the current total delay (packet buffer and sync buffer) in ms.
   virtual int CurrentDelayMs() const = 0;