[iOS][Getaway] Implement correct voiceover on the authentication screen.

Adds a screen title, makes the blocking screen a modal, and adds a
magic tap to start authentication.

I don't have the final string yet, the bug 1138892 is updated with the
new one that's required.

Bug: 1138892
Change-Id: If6312eb90a1686446f554f8713a1047bdb30256d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593265Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837552}

ios/chrome/browser/ui/incognito_reauth/incognito_reauth_view.mm

@@ -29,6 +29,10 @@ const CGFloat kButtonSpacing = 16.0f;
         [[UIVisualEffectView alloc] initWithEffect:blurEffect];
     [self addSubview:blurBackgroundView];
     blurBackgroundView.translatesAutoresizingMaskIntoConstraints = NO;
+    // TODO(crbug.com/1138892): add localized text.
+    blurBackgroundView.accessibilityLabel =
+        @"[Test String] Authenticate to access Incognito content";
+    blurBackgroundView.isAccessibilityElement = YES;
     AddSameConstraints(self, blurBackgroundView);
 
     _authenticateButton =
@@ -99,4 +103,16 @@ const CGFloat kButtonSpacing = 16.0f;
   return button;
 }
 
+#pragma mark - voiceover
+
+- (BOOL)accessibilityViewIsModal {
+  return YES;
+}
+
+- (BOOL)accessibilityPerformMagicTap {
+  [self.authenticateButton
+      sendActionsForControlEvents:UIControlEventTouchUpInside];
+  return YES;
+}
+
 @end