Change Chromium .gclient to not use Managed mode.

Since the sync_chromium.py script always passes --revision
to the gclient sync command, we don't need to have
managed=True in the .gclient file.
This will avoid a warning that confuses our developers.

BUG=3776
TESTED=Removed my chromium/.last_sync_chromium and performed
a gclient sync with this patch applied. No warning complaining
about Managed mode appears.

R=andresp@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23469004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7065 4adac7df-926f-26a2-2b94-8c16560cd09d