Reland "Move webrtc/{base => rtc_base} (stub headers)

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-Original-Commit-Position: refs/heads/master@{#18822}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f4efb6fb3d701616193d438934fa663a06b64b94
164 files changed