NetEq: Implement two UMA stats for delay adaptation.

This CL adds calculation and logging of average excess buffer delay
and number of delayed packet outage events per minute.

The first is the average of time spent in the packet buffer for all
packets. The average is calculated for intervals of one minute, and
the result is logged to the UMA stat
WebRTC.Audio.AverageExcessBufferDelayMs.

The second is a counter of delayed packet outage events that is
restarted every minute, and the result is logged to the UMA stat
WebRTC.Audio.DelayedPacketOutageEventsPerMinute. For a description of
delayed packet outages, see previous CL implementing a duration log
for these events.

BUG=webrtc:4915, chromium:488124
R=minyue@webrtc.org

Review URL: https://codereview.webrtc.org/1287333005 .

Cr-Commit-Position: refs/heads/master@{#9731}
2 files changed