Migrate PerformPostUserLoggedInActions into OnUserLoggedIn.

UserSessionManager::PerformPostUserLoggedInActions is directly
invoked from ChromeUserManagerImpl.
Because UserSessionManager is already observing UserManager's events,
this CL migrates the method into the observer method to split the
dependency from ChromeUserManagerImpl to UserSessionManager.

BUG=278643115
TEST=Tryjob

Change-Id: I582b9b42def9a830acd1407082127c50cf8ea256
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404310
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280188}