With this CL we finally actually set the root register to the correct
value.  Verification is still preserved by keeping a magic number in
IsolateData.

Bug: v8:6666
Change-Id: I89cb7cb36f977ac677ec33a814a2798baab4cec4
Reviewed-on: https://chromium-review.googlesource.com/c/1278277Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56627}