In BuildLoadNativeContextField(), we have access to the native context
as a constant. Use it directly, rather than loading from the current
context.

Change-Id: I60cd97586bbe5f8336f809967a3df556d0d43049
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1630682Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61871}