There's two considerations involving off-heap code and safepoint
tables.

1. Since the safepoint table is embedded within the instructions area
of code objects, we need to ensure that the actual instruction size
(i.e.  safepoint_table_offset if a code object has safepoints) is
large enough for the off-heap trampoline.

2. The pc-relative calculation in SafepointTable::FindEntry must be
able to handle off-heap pcs.

Bug: v8:6666
Change-Id: I92a5ecc49d0a78755b89c3c5774523afb21cd724
Reviewed-on: https://chromium-review.googlesource.com/934242Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51561}