ed9b9ff597
Reason for revert:
Breaks tests in downstream projects.
Original issue's description:
> Protect new header extension by field trial experiment to allow hardcoding it in SDP
>
> BUG=chrome:718738
>
> Review-Url: https://codereview.webrtc.org/2922683002
> Cr-Commit-Position: refs/heads/master@{#18409}
> Committed: cafa1d6bbe
TBR=sprang@webrtc.org,asapersson@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chrome:718738
Review-Url: https://codereview.webrtc.org/2922723002
Cr-Commit-Position: refs/heads/master@{#18414}