When the Isolate is already available, there is no need to fetch it
through GetIsolate.

Change-Id: I3ddc3d46924b401f154360564b203406a9b9e8e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3510389
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79439}