git: make fetch chromium use unmanaged mode
git: make fetch chromium use unmanaged mode

This has been determined to work in a better,
more predictable and robust way.

BUG=none
R=szager@google.com

Review URL: https://codereview.chromium.org/22852017

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@219121 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools