[iOS] Fixing authentification flow in Google services settings

Once the AuthenticationFlow is finished,
-GoogleServicesSettingsCoordinator.authenticationFlow needs to be set to
nil.

Change-Id: Ib9878141580dbfd2d7a70a81b0ac87c1255d67b7
Reviewed-on: https://chromium-review.googlesource.com/c/1409447Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622988}

ios/chrome/browser/ui/settings/google_services_settings_coordinator.mm

@@ -72,6 +72,12 @@
   [self.navigationController dismissViewControllerAnimated:YES completion:nil];
 }
 
+- (void)authenticationFlowDidComplete {
+  DCHECK(self.authenticationFlow);
+  self.authenticationFlow = nil;
+  [self.googleServicesSettingsViewController allowUserInteraction];
+}
+
 #pragma mark - Properties
 
 - (AuthenticationService*)authService {
@@ -101,7 +107,7 @@
   __weak GoogleServicesSettingsCoordinator* weakSelf = self;
   [self.authenticationFlow startSignInWithCompletion:^(BOOL success) {
     // TODO(crbug.com/889919): Needs to add histogram for |success|.
-    [weakSelf.googleServicesSettingsViewController allowUserInteraction];
+    [weakSelf authenticationFlowDidComplete];
   }];
 }