Iframes with `display: none;` shouldn't be required to have a title.

[romainmenke]

Google Tag Manager recently switched to using iframes to load a service worker.
These iframes don't have a title but do have style="display: none;"

I don't think these should be recorded as failures:

• https://www.w3.org/WAI/WCAG21/Techniques/html/H64
• https://www.w3.org/TR/WCAG21/#bypass-blocks

Suggested fix:

HTMLCS.Util.js

    /**
     * Returns true if the element is deliberately hidden from Accessibility APIs.
     *
     * @param {Node} element The element to check.
     *
     * @return {Boolean}
     */
    self.isAccessibilityHidden = function(element) {
        do {
            // WAI-ARIA presentation role.
            if (element.hasAttribute('role') && element.getAttribute('role') === 'presentation') {
                return true;
            }

            // WAI-ARIA hidden attribute.
            if (element.hasAttribute('aria-hidden') && element.getAttribute('aria-hidden') === 'true') {
                return true;
            }

+            // CSS hidden attribute.
+            var style = self.style(element);
+            if (style !== null && style.display === 'none') {
+                return true;
+            }
+
            // Consider iframes to be "invisible" as well, if the setting is set, so that
            // checks don't run for external widgets we don't have control over.
            if (element.tagName == 'IFRAME' && HTMLCS.getBosaOption('skipIframes') == true) {
                return true;
            }
        } while (element = element.parentElement);

        return false;
    };