[iOS] Fixing issue to interrupt AddAccountSigninCoordinator

When AddAccountSigninCoordinator is interrupted, the signinCompletion
needs to always be called. This was not the case when the coordinator
shows |identityInteractionManager|.

Bug: 971989
Change-Id: Ia7156313c5f5b723a847b6759db90fa9d5f83e31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144275Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758918}

ios/chrome/browser/ui/authentication/signin/add_account_signin/add_account_signin_coordinator.mm

@@ -69,6 +69,11 @@ using signin_metrics::PromoAction;
 - (void)interruptWithAction:(SigninCoordinatorInterruptAction)action
                  completion:(ProceduralBlock)completion {
   if (self.userSigninCoordinator) {
+    DCHECK(!self.identityInteractionManager);
+    // When interrupting |self.userSigninCoordinator|,
+    // |self.userSigninCoordinator.signinCompletion| is called. This callback
+    // is in charge to call |[self runCompletionCallbackWithSigninResult:
+    // identity:showAdvancedSettingsSignin:].
     [self.userSigninCoordinator interruptWithAction:action
                                          completion:completion];
     return;
@@ -90,6 +95,9 @@ using signin_metrics::PromoAction;
       [self.identityInteractionManager cancelAndDismissAnimated:NO];
       break;
   }
+  [self runCompletionCallbackWithSigninResult:SigninCoordinatorResultInterrupted
+                                     identity:nil
+                   showAdvancedSettingsSignin:NO];
   if (completion) {
     completion();
   }