This eases transition handlers caching and avoids memory overhead of
respective StoreHandler objects. In addition, it allows to use such
transition handlers on runtime side to make Object.assign implementation
a bit faster.

Bug: v8:5988
Change-Id: Iba660a11d4b300cd5f80615fb7e2608e53da8fee
Reviewed-on: https://chromium-review.googlesource.com/931701Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52187}