Move sent key frame stats to send_statistics_proxy class.

BUG=

Review URL: https://codereview.webrtc.org/1374673003

Cr-Original-Commit-Position: refs/heads/master@{#10166}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: dec5ebf10614115d6f2561c65f0cce3fd80ecfd2
10 files changed