Off-heap code cannot embed external references. With this CL, we load
from the external reference table (reached through the root pointer)
instead.

In a follow-up, the table could be stored within the isolate itself,
removing one more level of indirection.

Bug: v8:6666
Change-Id: I4c612ad3d4112ec03c3b389f5bfb9cdc3dc8a671
Reviewed-on: https://chromium-review.googlesource.com/970468
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52073}