... as opposed to prototype maps only.

This recovers the
JSTests/ObjectLiteralSpread/ObjectSpreadAndOverwrite regression.

Bug: chromium:930680, v8:8361
Change-Id: I71510f0ac76b19d16aaf3249a5cf533adf425a20
Reviewed-on: https://chromium-review.googlesource.com/c/1472632Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59616}