GN: Handle appending non-lists to the sources variable.

Previously this would assert because the thing that filtered lists assumed the thing being appended was a list.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/116143007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242263 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242263