PromiseCource(x) should return "not a thenable" if Type(x) is not Object
even if x.then exists.

BUG=347095
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/194403002

Patch from Yutaka Hirano <yhirano@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19788 ce2b1a6d-e550-0410-aec6-3dcde31c8c00