[iOS] Fixing AddAccountSigninCoordinator when being interrupted

When the coordinator is interrupted, the ChromeIdentityInteractionManager
needs to be dismissed. Once it is dismissed, the manager can be set to
nil.

Bug: 971989
Change-Id: I0da58b71e3e151d71e76fce5d9a8f53c00e66b72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154185Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759979}

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

@@ -95,6 +95,8 @@ using signin_metrics::PromoAction;
       [self.identityInteractionManager cancelAndDismissAnimated:NO];
       break;
   }
+  // The manager UI has been dismissed, it can be set to nil.
+  self.identityInteractionManager = nil;
   [self runCompletionCallbackWithSigninResult:SigninCoordinatorResultInterrupted
                                      identity:nil
                    showAdvancedSettingsSignin:NO];