[cr tool] Fixes detecting android downstream clients

The internal gclient doesn't have target_os.

BUG=

Review URL: https://codereview.chromium.org/110433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240121 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240121