Mutable references are allowed by the style guide and the linter has
been adjusted.

Bug: v8:10624, chromium:1056170
Change-Id: I8a7dc06da5758f5c714a5e61d75378c2a13ffb9b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2252192
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68455}