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

This is a reland of b0edf8e6

Bug: v8:8096
Change-Id: Ib2745d18e93788b92b0f9ffcaf996a1dad886b04
Reviewed-on: https://chromium-review.googlesource.com/1226875
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55953}