1. Download the Chromium depot tools
$svn co http://src.chromium.org/svn/trunk/tools/depot_tools
$export PATH=`pwd`/depot_tools:"$PATH"
2. Download the gitpackage
$apt-get install git
3. Get a copy of the Chromium OS repository
$mkdir [chromiumos]
$cd [chromiumos]
$gclient config http://src.chromium.org/git/chromiumos.git
$gclient sync