HeapObject::GetHeap and HeapObject::GetIsolate are gone since
https://crrev.com/c/1140319 (landed in July), hence we can clean up the
using declarations to force use of the methods declared in
NeverReadOnlySpaceObject.

R=delphick@chromium.org

Bug: v8:7786
Change-Id: Iec4edd394ac57a3f378dd1a9a100320e82cf8ea5
Reviewed-on: https://chromium-review.googlesource.com/1224414Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55863}