Make it explicit that AddShadowingKeys might allocate.

Bug: chromium:1049013
Change-Id: I938531a0324fa581422b74813518f3e85c9b3fbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2046888
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66288}