Removes uses of HeapObject::GetIsolate()/GetHeap() from
VerifyPointersVisitor by adding it to the visitor at construction time.

Bug: v8:7786
Change-Id: I28388f2eadbaf9947eafe0c62492c9a4781be250
Reviewed-on: https://chromium-review.googlesource.com/1071575Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53325}