Move RTP stats histograms from VieChannel to SendStatisticsProxy.

Also slice for screensharing.

BUG=
R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1734933002 .

Cr-Original-Commit-Position: refs/heads/master@{#11822}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 22c2b4814ae69d230512eb93cc20771fcf38879b
7 files changed