Add webrtc/rtc_base in preparation for the larger rename
in https://codereview.webrtc.org/2877023002.
This is needed in order to make downstream projects update
their dependencies (since it's not possible to use public_deps
for android_library).
BUG=webrtc:7634
NOTRY=True
Change-Id: I90fa72c9ebbc4e69bb3133413c3258432a5a72b1
Reviewed-on: https://chromium-review.googlesource.com/541215
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18699}
