chromeos: Only set primary account for new user logins

Today the code sets a primary account on every login. This should not
be necessary, but there is a comment saying it is done to ensure
the google service username is set. That code dates back to 2012 and
was apparently added for migration to a new pref. All users should be
migrated by now.

Remove the comment and only SetPrimaryAccount() for new logins.
Add CHECKs that a primary account exists for existing logins.

No behavior changes are expected.

See comments on this code review for details:
https://chromium-review.googlesource.com/c/chromium/src/+/2001018

Bug: none
Change-Id: I61e68c38a52a16b11508fb75fe0df8c9391645fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006261
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732676}

chrome/browser/chromeos/login/session/user_session_manager.cc

@@ -1415,16 +1415,18 @@ void UserSessionManager::InitProfilePreferences(
       // already present in |AccountManager|.
 
       // 2. Make sure that IdentityManager has been notified about it.
-      base::Optional<AccountInfo> maybe_account_info =
+      base::Optional<AccountInfo> account_info =
           identity_manager
               ->FindExtendedAccountInfoForAccountWithRefreshTokenByGaiaId(
                   gaia_id);
-      DCHECK(maybe_account_info.has_value());
-      // Make sure that the google service username is properly set (we do this
-      // on every sign in, not just the first login, to deal with existing
-      // profiles that might not have it set yet).
-      identity_manager->GetPrimaryAccountMutator()->SetPrimaryAccount(
-          maybe_account_info->account_id);
+      DCHECK(account_info.has_value());
+      if (user_manager->IsCurrentUserNew() || profile->IsNewProfile()) {
+        identity_manager->GetPrimaryAccountMutator()->SetPrimaryAccount(
+            account_info->account_id);
+      } else {
+        CHECK(identity_manager->HasPrimaryAccount());
+        CHECK_EQ(identity_manager->GetPrimaryAccountInfo().gaia, gaia_id);
+      }
     } else {
       // Make sure that the google service username is properly set (we do this
       // on every sign in, not just the first login, to deal with existing