[iOS] Adding comment in main controller

Adding comment to explain when the SigninInteractionCoordinator needs
to be dismissed -[MainController closeSettingsAnimated:completion:].

Related to:
crrev.com/c/1788026/4/ios/chrome/app/main_controller.mm#2275

Change-Id: Ie7685cadf00f9d7893ce55fb346667f014272d15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795904Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695171}

ios/chrome/app/main_controller.mm

@@ -2247,6 +2247,11 @@ enum class EnterTabSwitcherSnapshotResult {
   // to be cloase, it is thus the responsibility of the main controller to
   // dismiss the the advanced sign-in settings by dismssing the settings
   // presented by |self.signinInteractionCoordinator|.
+  // To reproduce this case:
+  //  - open Bookmark view
+  //  - start sign-in
+  //  - tap on "Settings" to open the advanced sign-in settings
+  //  - tap on "Manage Your Google Account"
   DCHECK(self.signinInteractionCoordinator.isSettingsViewPresented);
   [self.signinInteractionCoordinator
       abortAndDismissSettingsViewAnimated:animated