This CL fixes two bugs. First, when looking for a free spot while
moving elements to the front, the prototype chain was also considered,
even though an object at a specific index might have a hole (free
spot).

Second, when moving an element to the front, we are not allowed to
delete it immediately (to preserve semantics when interacting with
non-extensible objects). Such an element is then a free spot, but
won't be recognised as such. This CL sets that element to undefined
after it was moved, to mark it as a free spot.

R=jgruber@chromium.org

Bug: chromium:897512,v8:8369
Change-Id: I79207215b8b0a3c714f064450d8fe5ca0ea4a096
Reviewed-on: https://chromium-review.googlesource.com/c/1417171
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58878}