SplitSettings: Sync 'Advanced' expansion state of expand only

Port this behavior from browser settings to OS settings:
https://chromium-review.googlesource.com/c/chromium/src/+/1610879

The tests are not ported yet because we're still setting up our test
scaffolding for the OS settings pages: crbug.com/955731

Bug: 950007
Change-Id: I22f7727e38750a7771de6fd49a608bf965a7f3b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629210
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663262}
2 files changed