This extends the Promise#then protector to also guard the intrinsic
%ObjectPrototype%, making it usable for fast-path logic in the
AsyncGeneratorResolve operation, where we can skip the "then" lookup
on the iteratorResult in that case. We also add a corresponding fast-
path to the ResolvePromise builtin itself, which avoids the second
"then" lookup on the async iterator side.

This reduces execution time of the fibonacci-async-es2017-native test
from 298.16ms to 280.55ms, which corresponds to an improvement of ~6%
in this case.

This is a rebased reland of

  https://chromium-review.googlesource.com/967203

which landed earlier, but had to be reverted as part of the mega-revert
that was necessary to fix the async_hooks breakage in Node 10.

Bug: v8:7253
Change-Id: Id9dd1ddc4a8285d64e92cd2030f3168a12e4b509
Reviewed-on: https://chromium-review.googlesource.com/1130523
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54352}