Port f7f18b0f

Original Commit Message:

    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.

R=jgruber@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I447c880c7f840a012e53870e28e2bd3bfa552040
Reviewed-on: https://chromium-review.googlesource.com/c/1393288
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58499}