Both the top_ pointer and the top_on_previous_step_ pointer can be one
byte beyond the current page. Page::FromAddress call should take that
into account.

Bug: chromium:777177
Change-Id: I9cbb5bc6eab932afc6d0c915fd70a9a7b20ba62c
Reviewed-on: https://chromium-review.googlesource.com/738204
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48962}