[iOS] Correctly use child view controller for alert overlays

The wrong class selector was mistakenly used when refactoring.  This CL
updates to use the correct selector that is queried by the presentation
context.

Bug: none
Change-Id: Iff711ef6f5f7261edcb37bca87922ef7ae729238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981116
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727286}

ios/chrome/browser/ui/overlays/common/alerts/alert_overlay_coordinator.mm

@@ -69,7 +69,7 @@
   return NO;
 }
 
-+ (BOOL)usesChildViewController {
++ (BOOL)showsOverlayUsingChildViewController {
   return YES;
 }