hbos and hta are already OWNERS of webrtc/stats/ and of rtcstats* files
(per-file rtcstats*=) in webrtc/api/. When the webrtc/api/stats/ folder
was created we forgot to add this OWNERS file (per-file OWNERS does not
apply to subfolders apparently).
BUG=chromium:627816
NOTRY=True
Review-Url: https://codereview.webrtc.org/2392633002
Cr-Commit-Position: refs/heads/master@{#14482}
