Add SetPrimaryProfilePath to set the primary profile.

Add the SetPrimaryProfilePath interface to set the primary user profile
for FullRestoreReadHandler. Because Lacros runs for the primary user
only, so we need to check whether the profile path is for the primary
user to decide whether we need lacros_read_handler_ to deal with it.

BUG=1239984

Change-Id: Ib8d0fec0d31a44fb1627bd0a0d8e76f622764d98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3403503


Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962214}