add some comments about DEPS lkgr for chromium
BUG=none
TESTED=none
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/16209005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6821 4adac7df-926f-26a2-2b94-8c16560cd09d