Refactor and clean-up relating to RTCCodecStats.

Refactor how |codec_id| is set, remove outdated TODO, update comments
with new bugs IDs.

BUG=webrtc:7061

Review-Url: https://codereview.webrtc.org/2670343002
Cr-Original-Commit-Position: refs/heads/master@{#16467}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 585a9b191c6b3f7c49fff471435dc65ffcb188ea
2 files changed