Make sync_chromium use the git-cache when on the bots.

This should help bootstrapping speed, as well as allow better clobbering
support.

R=agable@chromium.org
TBR=agable@chromium.org, kjellander@webrtc.org, szager@chromium.org
BUG=2863, chromium:339647

Review URL: https://webrtc-codereview.appspot.com/17179004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6943 4adac7df-926f-26a2-2b94-8c16560cd09d