Moves warning about invalid targets into 'invalid_targets' in result

Previously if an invalid_target was supplied analyze would report it
via 'warning'. I'm moving that warning to 'invalid_targets' and
providing the list of invalid targets there.

BUG=426892
TEST=covered by tests
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/671233004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1994 78cadc50-ecff-11dd-a971-7dbc132099af