Add statistics gathering for packet loss.

Adds a class used to classify whether packet loss events are a single packet or multiple packets as well as how many packets have been lost. Also exposes a new function in the RtpRtcp interface to retrieve these statistics.

BUG=

Review URL: https://codereview.webrtc.org/1198853004

Cr-Commit-Position: refs/heads/master@{#9568}
11 files changed