Only read-only pages don't have properly initialized reservation object.

Bug: v8:8096
Change-Id: I83f4baa414dc2ca5a397a9897088060b6cac4783
Reviewed-on: https://chromium-review.googlesource.com/1216562Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55763}