Improves output of git cl tree to use Chromium OS status, not Chromium Status.

BUG=chromium-os:11449
TEST=Output of 'git cl tree' for Chromium OS includes output from Chromium OS instead of from Chromium.

Review URL: http://codereview.chromium.org/6312047

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@73292 0039d316-1c4b-4281-b951-d872f2087c98