Revert 6839 "Allow root build dependencies to be overridden."

> Allow root build dependencies to be overridden.
> 
> RISK=P2
> TESTED=manual
> R=andrew@webrtc.org, thorcarpenter@google.com
> 
> Review URL: https://webrtc-codereview.appspot.com/19009004

TBR=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/20099004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6840 4adac7df-926f-26a2-2b94-8c16560cd09d