Off-heap-safety slightly differs from isolate-independence in that it
allows external references and checks instruction-size constraints.

This adds the new predicate as well as a cctest verifying it. New
DCHECKs are introduced to document assumptions and upcoming work.

Note that this breaks the --stress-off-heap-code flag. Fixes will
follow in upcoming CLs.

Bug: v8:6666
Change-Id: If4f3e0f4428bacc8d293cd864b9b07b81679c423
Reviewed-on: https://chromium-review.googlesource.com/934183
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51513}