Add the stub headers from https://codereview.webrtc.org/2877023002 as a separate commit. This preserves git blame history of the moved files. BUG=webrtc:7634 NOTRY=True TBR=kwiberg@webrtc.org Change-Id: Ic141abf11801fbfdeea5bcdb23608696ad449013 Reviewed-on: https://chromium-review.googlesource.com/554623 Reviewed-by: Henrik Kjellander <kjellander@webrtc.org> Cr-Commit-Position: refs/heads/master@{#18822}