Move UserManager observation to ChromeSessionManager

UserSessionManager is singleton, so it is not easy to handle
its lifetime in unit tests. This CL moves UserManager's observation part
from UserSessionManager to ChromeSessionManager, so we have more control
of its lifetime, specifically in unittests.

BUG=278643115
TEST=Tryjob

Change-Id: Ib8201915d4f29f489b40ae083093668bb6bf0ee9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5409960


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281547}