This patch also refactors most of PromiseThen into InternalPromiseThen to
be reused with PromiseCatch and also changes InternalResolvePromise to
return and not branch.

BUG=v8:5343

Review-Url: https://codereview.chromium.org/2596553002
Cr-Commit-Position: refs/heads/master@{#41902}