[iOS][Safety Check] Add button accessibility label

Add button accessiblility label to items when they are clickable.

Bug: 1078782
Change-Id: I07fb2e6b965218aefd4fb5ff0627ccfa5c202834
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490108
Commit-Queue: Sean Harrison <harrisonsean@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820247}

ios/chrome/browser/ui/settings/safety_check/safety_check_mediator.mm

@@ -279,6 +279,7 @@ constexpr double kSafeBrowsingRowMinDelay = 1.75;
         [[TableViewTextItem alloc] initWithType:CheckStartItemType];
     _checkStartItem.text = GetNSString(IDS_IOS_CHECK_PASSWORDS_NOW_BUTTON);
     _checkStartItem.textColor = [UIColor colorNamed:kBlueColor];
+    _checkStartItem.accessibilityTraits |= UIAccessibilityTraitButton;
   }
   return self;
 }