HeapVisitor is a simple visitor implementing both visitation and accept
interfaces. HeapVisitor is aimed to replace functions that traverse the
heap in Blink: PrepareForSweep, Sweep, FinalizeSweep, MakeConsistentForGC,
ObjectPayloadSize, VerifyObjectStartBitmap, CollectStatistics, etc.

This CL also introduces a Heap::ObjectPayloadSize function.

Bug: chromium:1056170
Change-Id: I85a2b8b572486bea29704fc436b002d8f7405201
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2165733
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67458}