This TODO clarifies intentions of 49f60a3b.

Bug: v8:9533
Change-Id: Ide448ae863a8716dafdcdd7fb27801a51655650e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2022145Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Steve Blackburn <steveblackburn@google.com>
Cr-Commit-Position: refs/heads/master@{#65997}