[iOS] Fix DCHECK in settings_table_view_controller.mm

EGTest added in crrev.com/i/4471147

Bug: 1288024
Change-Id: I492483217f5de49d0e491ad97602fb4b82ebb02b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3395674
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/main@{#960111}

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

@@ -1894,10 +1894,13 @@ - (void)onPreferenceChanged:(const std::string&)preferenceName {
 - (void)configureSigninPromoWithConfigurator:
             (SigninPromoViewConfigurator*)configurator
                              identityChanged:(BOOL)identityChanged {
-  DCHECK(!self.isSigninInProgress);
-  if (![self.tableViewModel
+  if (self.isSigninInProgress ||
+      ![self.tableViewModel
           hasItemForItemType:SettingsItemTypeSigninPromo
            sectionIdentifier:SettingsSectionIdentifierSignIn]) {
+    // Don't reload the sign-in promo if sign-in is in progress, to avoid having
+    // UI glitches. The table view should be reloaded once the sign-in is
+    // finished.
     return;
   }
   NSIndexPath* signinPromoCellIndexPath = [self.tableViewModel