[iOS] Disallow use of UserSigninCoordinator for signed-in users.

Adds a comment explaining the flexibility of the AuthenticationFlow
service and how this may cause issues in sign-in UI expectations.

Bug: 1125631
Change-Id: Ia2f5aa12a7092c57a2c91f8eae1df6079ddd980d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461345
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816501}

ios/chrome/browser/ui/authentication/signin/user_signin/user_signin_coordinator.mm

@@ -7,6 +7,7 @@
 
 #import "base/mac/foundation_util.h"
 #import "ios/chrome/browser/main/browser.h"
+#import "ios/chrome/browser/signin/authentication_service.h"
 #import "ios/chrome/browser/signin/authentication_service_factory.h"
 #import "ios/chrome/browser/signin/identity_manager_factory.h"
 #import "ios/chrome/browser/sync/consent_auditor_factory.h"
@@ -100,6 +101,12 @@ const CGFloat kFadeOutAnimationDuration = 0.16f;
 #pragma mark - SigninCoordinator
 
 - (void)start {
+  // The user should be signed out before triggering sign-in or upgrade states.
+  // Users are allowed to be signed-in during FirstRun for testing purposes.
+  DCHECK(!AuthenticationServiceFactory::GetForBrowserState(
+              self.browser->GetBrowserState())
+              ->IsAuthenticated() ||
+         self.signinIntent == UserSigninIntentFirstRun);
   [super start];
   self.viewController = [[UserSigninViewController alloc] init];
   self.viewController.delegate = self;