Add three WebRTC audio jitter buffer metrics to histograms
Add three WebRTC audio jitter buffer metrics to histograms

This change adds three new metrics that all relate to how well the
delay adaptation in WebRTC's audio jitter buffer (a.k.a. NetEq)
works. The metrics are:

- WebRTC.Audio.AverageExcessBufferDelayMs
- WebRTC.Audio.DelayedPacketOutageEventMs
- WebRTC.Audio.DelayedPacketOutageEventsPerMinute

The calculation and logging of the metrics are introduces in the
following CLs:

https://codereview.webrtc.org/1290113002/
https://codereview.webrtc.org/1287333005/

BUG=webrtc:4915, chromium:488124

Review URL: https://codereview.chromium.org/1302503002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#344211}

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