Portals: Support adoption of the predecessor.

This CL adds support for an activated portal to adopt its predecessor
as a portal. Adopting the predecessor enables developers to quickly
transition back and forth between different pages.

Bug: 914122

Change-Id: I80381017ddb774da20ae07efcba2f4fee733fc3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1456869
Commit-Queue: Lucas Gadani <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Jeremy Roman <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#643463}
27 files changed