Drive-by-fix: Add V8_WARN_UNUSED_RESULT to MaybeHandle::ToHandle

Bug: chromium:1057653
Change-Id: I2834806ca498a2fa43a64f5391606cdbfb4af4fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2084814Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66582}