This allows updates to SLOW_STRING_WRAPPER_ELEMENTS elements kind
for non-extensible map (so far only dictionary elements and typed-array
elements were allowed).

Bug: chromium:932101
Change-Id: Id532684aa94f908eb14f3451aa823f282342668b
Reviewed-on: https://chromium-review.googlesource.com/c/1475390Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59621}