External references located within the isolate can be loaded as a
fixed offset from the root register. This avoids one load from memory
over the default indirect method (which reads from the external
reference table).

Bug: v8:6666
Change-Id: I569308c03f1a8c3feb1be7ceb072178b5be17b25
Reviewed-on: https://chromium-review.googlesource.com/1092739
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53661}