Add UMA metric to track usage of sending a mousedown to select elements.

third_party/WebKit/LayoutTests/imported/wpt/custom-elements/resources/custom-elements-helpers.js

+function create_window_in_test(t, srcdoc) {
+  let p = new Promise((resolve) => {
+    let f = document.createElement('iframe');
+    f.srcdoc = srcdoc ? srcdoc : '';
+    f.onload = (event) => {
+      let w = f.contentWindow;
+      t.add_cleanup(() => f.parentNode && f.remove());
+      resolve(w);
+    };
+    document.body.appendChild(f);
+  });
+  return p;
+}
+
+function test_with_window(f, name, srcdoc) {
+  promise_test((t) => {
+    return create_window_in_test(t, srcdoc)
+    .then((w) => {
+      f(w);
+    });
+  }, name);
+}