GN: Prepare to remove webrtc_base target

Keep the webrtc_base target temporarily while waiting for
Chromium to pick up this revision. Then we'll update Chromium
and remove the webrtc_base target for real.

This should have been a part of https://code.google.com/p/webrtc/source/detail?r=7140

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/33979004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8117 4adac7df-926f-26a2-2b94-8c16560cd09d