There are only two users of hidden prototypes left and they both only have 1 level. This slightly simplifies the handcrafted code.

Bug: v8:5561
Change-Id: I674e72f1465ccbe75c0bb63f7eea3525830145cb
Reviewed-on: https://chromium-review.googlesource.com/512745Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45503}