Revert "Move webrtc/{base => rtc_base}" (https://codereview.webrtc.org/2877023002)

Will reland in two different commits to preserve git blame history.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: I550da8525aeb9c5b8f96338fcf1c9714f3dcdab1
Reviewed-on: https://chromium-review.googlesource.com/554610
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18820}

PRESUBMIT.py

@@ -32,7 +32,6 @@ CPPLINT_BLACKLIST = [
   'webrtc/modules/video_capture',
   'webrtc/p2p',
   'webrtc/pc',
-  'webrtc/rtc_base',
   'webrtc/sdk/android/src/jni',
   'webrtc/sdk/objc',
   'webrtc/system_wrappers',