This makes an ObjectVisitor as powerful as a StaticVisitor and allows
slots recording in ObjectVisitor.

This patch also renames VisitCell method of ObjectVisitor to
VisitCellPointer, so that VisitCell is free to be used for actually
visiting a cell.

BUG=chromium:709075

Review-Url: https://codereview.chromium.org/2810653002
Cr-Commit-Position: refs/heads/master@{#44860}