Original issue's description:
> Prepare to enable in-object properties in subclasses on a case by case basis.
>
> Minor cleanup in VisitorId selection.
>
> Committed: https://crrev.com/7c449a62edfc03aed84d94da323dcfe2b51a3600
> Cr-Commit-Position: refs/heads/master@{#32030}

This is a mostly clean reland.

Review URL: https://codereview.chromium.org/1459133002

Cr-Commit-Position: refs/heads/master@{#32148}