Fixes some failing test262 tests for a corner-case in the spec
where we need to use the buffer constructor from a different realm.

Bug: v8:7512
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I3f1334f6181eaaddf0326156139ac20a970c235b
Reviewed-on: https://chromium-review.googlesource.com/966223
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52560}