Rename some variables and methods in RTC event log.

Rename loss based and delay based bwe updates in proto (and correspondingly in the C++ code).

BUG=webrtc:6423

Review-Url: https://codereview.webrtc.org/2705613002
Cr-Original-Commit-Position: refs/heads/master@{#16719}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 424e6cfd589e0cdbfc9a664644dd1867271245f7