This makes the above factory function use a fine-grained scope when
unlocking code space for modification. It now uses a cumulative per-page
scope that collects only those memory chunks that needed to be touched
during allocation.

R=ulan@chromium.org

Change-Id: I22de0fae8be507ba9d51868f668b902b3bae3d6a
Reviewed-on: https://chromium-review.googlesource.com/c/1335558Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57503}