When installing getter/setter of non-extensible map with existing
setter/getter of the same name, we introduce a new transition
(so we have two transitions with the same name!). This triggers
an assertion in map updater.

This fix carefully checks that on the back-pointer path from
non-extensible map to the extensible map there are only
integrity level transitions. Otherwise, we just bail out.

Bug: chromium:932953
Change-Id: I02e91c3b652428a84a9f5c58b6691ea9b1fc44d6
Reviewed-on: https://chromium-review.googlesource.com/c/1477067Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59667}