We no longer need the kAlreadyVisitedSlot because we can just check
for undefined in the kPromiseSlot to know if the clsoure was already
fulfilled.

This means we save one word per context per promise resolving closure.

Bug: v8:7037
Change-Id: Ib8f0fb445d2e143714d57fe644ba6d7a3f04c1f7
Reviewed-on: https://chromium-review.googlesource.com/756176Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49200}