NativeContext will soon outgrow the limits of the fixed sized map.
This CL simply moves NativeContext back to the variable sized map.

Bug: v8:9463
Change-Id: I477dc5f19ed22b5b2b8d3415daad9d87e785bdcb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1774185Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63524}