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

Reland the base->rtc_base without adding stub headers (will be
done in follow-up CL). This preserves git blame history of all files.

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

Change-Id: Iea3bb6f3f67b8374c96337b63e8f5aa3e6181012
Reviewed-on: https://chromium-review.googlesource.com/554611
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18821}

PRESUBMIT.py

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