Add UMA for send bwe and pacer bitrate.

Review URL: https://codereview.webrtc.org/1434403004

Cr-Original-Commit-Position: refs/heads/master@{#10675}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 18adf0a79d4a0ea124c7f27fd553382d0b0cb7ac
2 files changed