In preparation for converting JSEntry into a builtin. The root register
used to be initialized by an embedded external reference, which is not
isolate-independent. Pass in its value explicitly instead.

Tbr: clemensh@chromium.org
Bug: v8:7777
Change-Id: If9d20cb193af175bd1cf58e5826bdf6f397869ad
Reviewed-on: https://chromium-review.googlesource.com/c/1363139Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58065}