Add warning comment Review URL: https://webrtc-codereview.appspot.com/933012
Add warning comment
Review URL: https://webrtc-codereview.appspot.com/933012

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: c79505f95e7db39a909929fc334bb78353e5e36c

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc