This enables the invariant that a NativeModule's code may either be
executable or writable, but never both at the same time.

Bug: v8:7105
Change-Id: If2abfce6796a365bb675a82140f32e8f45bb923f
Reviewed-on: https://chromium-review.googlesource.com/804208
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49840}