Fix broken Promise-related layout tests.

Because of the bindings change related to Promise-returning functions (see
http://crbug.com/359411), some layout tests that had been written before that
have been broken. As the bindings change is done, this CL fixes these tests.

BUG=362992

Review URL: https://codereview.chromium.org/669353002

git-svn-id: svn://svn.chromium.org/blink/trunk@184319 bbb929c8-8fbe-4397-9dbb-9b2b20218538