A DCHECK in LookupIterator::name hits when we add a indexed property,
as it requires a named property.
This replaces it with GetName to avoid the failure.

Bug: chromium:959727
Change-Id: I1e98b313ec9257db80460a34d691016acbceb3c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1597372
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61358}