This CL introduces the following visitors:
1) RecordMigratedSlotVisitor which simplifies MarkCompactCollector::MigrateObject().
2) IteratePointersToFromSpaceVisitor which simplifies Heap::IteratePointersToFromSpace().
3) FindPointersToNewSpaceVisitor which simplifies StoreBuffer::IteratePointersToNewSpace().

These changes make the object's body descriptors the one and only place that knows how to traverse the object.

Review URL: https://codereview.chromium.org/1441453002

Cr-Commit-Position: refs/heads/master@{#31992}