Now that we've removed kRootRegister from all ia32 interface
descriptors, let's make sure it does not sneak back in.

Bug: v8:6666
Change-Id: Ie3528908a142c36f106b0053041ed974216533d4
Reviewed-on: https://chromium-review.googlesource.com/1202083
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55590}