Update checkdeps.py rules in DEPS

The initial rules didn't allow including
source from third_party, which is incorrect.
Cleanup irrelevant rules for directories that
are ignored, since WebRTC don't have any source
code in those locations.

BUG=
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/30599004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7351 4adac7df-926f-26a2-2b94-8c16560cd09d