Since the StubCache it's cleared at the end of the GC, it doesn't
matter if it contains weak or strong pointers.

BUG=v8:7308

Change-Id: Ib141e3d411523c67ccb8f8979845a88488d6e4ee
Reviewed-on: https://chromium-review.googlesource.com/1064053
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53311}