Previously it was allowed to call AddStats with stats of the same ID
multiple times.
This revealed a few things:
- Local and remote streams can have the same label.
RTCMediaStreamStats's ID is updated to include "local"/"remote".
- The same certificate can show up multiple times (e.g. for local and
remote in a loopback), so we skip creating RTCCertificateStats for the
same certificate multiple times
BUG=chromium:627816
Review-Url: https://codereview.webrtc.org/2593503003
Cr-Commit-Position: refs/heads/master@{#15730}
02d2a92d92