PromiseThen should ignore non-function parameters.

When non-function parameters are given, PromiseThen should work as if
undefined parameters were given.

BUG=347455
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/180723011

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21025 ce2b1a6d-e550-0410-aec6-3dcde31c8c00