Rewrote pacer and bandwidth UMA stats.
Rewrote pacer and bandwidth UMA stats.

The new version measures receive bitrates from time of first packet to
time of last packet, and send/pacer BWE as the average BWE reported
while we have send streams.

R=asapersson@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1470373004 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10810}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 226befecfb5e56287482a2d6250f01d019761884

jchavanton/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk