Makes it easier to modify JSFunction; the failing asserts will tell which places
need to be modified.

Change-Id: I70c7cd0b89cf05964b4730990fb3d7f23927437a
Reviewed-on: https://chromium-review.googlesource.com/781689Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49732}