This makes the above factory function use a fine-grained scope when
unlocking code space for modification. It is now based on the memory
chunk of the resulting code object.

R=ulan@chromium.org

Change-Id: Iabe6fba7595ba3264b21bcd6f6634ab9725eaad9
Reviewed-on: https://chromium-review.googlesource.com/c/1335687Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57498}