Reduce the code duplication overhead in the InternalPerformPromiseThen
helper, which saves quite a bit of space and makes code more concise and
readable.

Bug: v8:7253, v8:7310
Change-Id: I64d11661d7258ced32df564d2e83c5ea45955415
Reviewed-on: https://chromium-review.googlesource.com/880722Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50800}