Instead, use Is<NativeContext> then UnsafeCast.

Bug: v8:9679
Change-Id: Id87a10fa66ff1c1e0ca0099b1938c72da2437832
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2631554
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72175}