Object.assign should not normalize JSGlobalProxy objects.

Bug: chromium:1139769
Change-Id: Ie7e24f6498267966b7553b0c5994307f5b632b0d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2485505Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70713}