It's possible in gclient to list multiple operating systems on the
command line and while handling them all perfectly might not be possible
it's possible to make it better than today.

This patch makes sure None never overrides any previous value and it
adds some output to indicate what kind of dangerous choices it made.

BUG=248168

Review URL: https://codereview.chromium.org/23875029

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@240892 0039d316-1c4b-4281-b951-d872f2087c98