Use webrtc_root instead of DEPTH for iSAC.

Un-breaks chromium.webrtc.fyi. Broken as Chromium doesn't have webrtc/
checked out in root.

TBR=bjornv@webrtc.org,tommi@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/28289004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7897 4adac7df-926f-26a2-2b94-8c16560cd09d