[iOS][Alert] Centers Alert to safe area

Uses the center constrains of the view's safe area instead of the view
ones.

R=kkhorimoto@chromium.org

Bug: 951300
Change-Id: If97c99f1d14dc4545833b9316364834f3862bb18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1594712Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658992}

ios/chrome/browser/ui/alert_view_controller/alert_view_controller.mm

@@ -126,9 +126,9 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
 
     // Centering
     [self.contentView.centerXAnchor
-        constraintEqualToAnchor:self.view.centerXAnchor],
+        constraintEqualToAnchor:self.view.safeAreaLayoutGuide.centerXAnchor],
     [self.contentView.centerYAnchor
-        constraintEqualToAnchor:self.view.centerYAnchor],
+        constraintEqualToAnchor:self.view.safeAreaLayoutGuide.centerYAnchor],
 
     // Minimum Size
     [self.contentView.heightAnchor