Add v8::Promise::Then.

Blink needs v8::Promise::Then to implement ScriptPromise::then.
Blink-side CL: https://codereview.chromium.org/316453002

BUG=371288
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/314553002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21805 ce2b1a6d-e550-0410-aec6-3dcde31c8c00