Currently v8::Promise::Then takes only one handler. It should take two handlers,
on_fulfilled and on_rejected like ECMAScript Promise.

Bug: chromium:912848
Change-Id: I08a20990a27b3f8621225fad42a8de1dad67796f
Reviewed-on: https://chromium-review.googlesource.com/c/1375509Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58204}