sync_chromium.py: Check for chromium/src

Make sure the script alwyas downloads Chromium
if there's no current download. This case can happen
if a user is removing the 'src' folder but doesn't know
to remove the .last_sync_chromium file.

BUG=
TESTED=Renamed chromium/src and ran a sync keeping the .last_sync_chromium file, verified it started downloading.
TBR=iannucci@chromium.org

Review URL: https://webrtc-codereview.appspot.com/27099004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7738 4adac7df-926f-26a2-2b94-8c16560cd09d