[iOS] Mark the Browser as modal for accessibility

This CL sets accessibilityViewIsModal to YES on the Browser view when
it is shown from the TabGrid. This makes sure that the elements of the
TabGrid aren't accessible with VoiceOver while the Browser is shown.

Fixed: 1067255
Change-Id: Ie01927a6170fbe5d7b8f1180c08f1cc4459d8849
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142271
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757450}

ios/chrome/browser/ui/tab_grid/transitions/tab_grid_transition_handler.mm

@@ -100,6 +100,8 @@ const CGFloat kReducedMotionDuration = 0.25;
   browser.view.frame = tabGrid.view.bounds;
   [tabGrid.view addSubview:browser.view];
 
+  browser.view.accessibilityViewIsModal = YES;
+
   browser.view.alpha = 0;
 
   if (UIAccessibilityIsReduceMotionEnabled() ||