Add SentToInputFpsRatioPercent UMA metric on send side

New metric is a ratio between SentFramesPerSecond and
InputFramesPerSecond. It's needed to measure how much of input frames
are actually sent.

BUG=webrtc:8178

Review-Url: https://codereview.webrtc.org/3010723002
Cr-Original-Commit-Position: refs/heads/master@{#19596}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f4ded682e913d56bbadf14a391ff3ba37a7419f5
1 file changed