In preparation for kRootRegister support on ia32.

Bug: v8:6666
Change-Id: I6bbc87734d189bb8cde5d057a54f8155606d142d
Reviewed-on: https://chromium-review.googlesource.com/1188319
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55406}