This generally shouldn't hit since prototypes are supposed to be fast; but it makes it more uniform anyway.
This CL also drops the limitation not supporting keyed loads with normal access. There's no point in returning the slow stub there.

BUG=

Change-Id: I5578269f6c64762610e885740a692d1e91aa6aa2
Reviewed-on: https://chromium-review.googlesource.com/447698
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43508}