d8 throws on unhandled rejected Promises since
https://crrev.com/c/2238569 so no special handling beyond throwing in
the async hooks themselves is needed.

Drive-by-fix: Use v8::Isolate* as local variable.

Bug: chromium:1238467
Change-Id: I271720cd9cfd1d30b58b5407c700b0f730910968
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3090333
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76255}