[iOS][MF] Post an a11y notification when presenting

Bug: 845472
Change-Id: I1691063eecb6fa423bc36c21a910adcf21f00da8
Reviewed-on: https://chromium-review.googlesource.com/c/1338108Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608468}

ios/chrome/browser/autofill/form_input_accessory_view_controller.mm

@@ -103,6 +103,7 @@ CGFloat const kInputAccessoryHeight = 44.0f;
   view.translatesAutoresizingMaskIntoConstraints = NO;
   AddSameConstraints(view, constrainingView);
   self.keyboardReplacementView = view;
+  UIAccessibilityPostNotification(UIAccessibilityLayoutChangedNotification, view);
 }
 
 #pragma mark - FormInputAccessoryConsumer