This was not implemented so far, leading to lots of clusterfuzz issues.
Testing this feature would require complicating the interface a lot and
exposing more implementation details in the header file, hence we just
go with regression tests for now.

R=ahaas@chromium.org, titzer@chromium.org

Bug: v8:6600, chromium:782280
Change-Id: I12863f3eb59a8dffdcc7d3bfb8e1f0ae0eec15ee
Reviewed-on: https://chromium-review.googlesource.com/758772Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49286}