These helpers support off-heap code objects, for which they return
start, end, and size of the off-heap instruction stream.

Bug: v8:6666
Change-Id: Ib5e819d976eee3073b0354b8d8ce324b691f1b15
Reviewed-on: https://chromium-review.googlesource.com/937281
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51572}