... instead of checking if the property cell is still empty when loading/storing
through JSGlobalObject prototype.

Also invalidate the validity cell when new global lexical variables appear in the
script.

Bug: v8:5561
Change-Id: Iaf122dffe76d57b32e2b69291dee079e772b271c
Reviewed-on: https://chromium-review.googlesource.com/819230Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50100}