The field type can always be generalized in-place, even for root maps.
There's no need to create defensive copies in this case. This happens
for example in the superWrapper in the Speedometer2/EmberJS-Debug test.

Bug: v8:8834, v8:9114
Change-Id: Ief05516d0e9d84850c52d2cf8815953ef77d5223
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1562131
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60778}