[iOS][Alert] Sets view as an accessibillity modal

This makes Voice Over not read the siblings views of the alert when
presented. That is the content behind the chrome and the alert.
Everything outside or above the alert view will be read.

Bug: 959764, 951300
Change-Id: Idb932e1fcc251899bccbb7b834e022432c740973
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611607
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659487}

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

@@ -112,6 +112,7 @@ constexpr int kTextfieldBackgroundColor = 0xf7f7f7;
   [super loadView];
   self.view.backgroundColor =
       [[UIColor blackColor] colorWithAlphaComponent:kBackgroundAlpha];
+  self.view.accessibilityViewIsModal = YES;
 
   self.tapRecognizer = [[UITapGestureRecognizer alloc]
       initWithTarget:self