context_name pointer can be changed after GC triggered by AddProperty.

R=ishell@chromium.org

Bug: chromium:732717
Change-Id: Ie8e2497fa9f3bac80e0ad68153956e382731e284
Reviewed-on: https://chromium-review.googlesource.com/532994
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45898}