Remove the native context slot from contexts by making context maps
native-context-specific. Now we require 2 loads to go from a context to the
native context, but we have 1 field fewer to store when creating contexts.

Change-Id: I3c0d7c50c94060c4129db684f46a567de6f30e8d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859629
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64296}