Implement similar functionality in the unit test which used this field.
One test gets slightly weaker by this.

R=mstarzinger@chromium.org

Bug: v8:8238
Change-Id: I0b047ff54f08a4549a2f78af30e21296bb1ee63f
Reviewed-on: https://chromium-review.googlesource.com/c/1327042
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57403}