The method has been non-const since initially introduced in de070ccf.
Here's a minor change to make it const, for it may/shall be, for other
similar methods are, and for making it easier for future changes
(accessing this method in another const method, etc.).

Change-Id: I3449214fc086403fc9b24b6f502ca20ac0b1426a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2055123Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66270}