Add a findActiveElement test helper that also examines shadow roots

The browser has one activeElement. Add a test helper that is similar to
getActiveElement but searches shadow roots for the actual activeElement
if needed, and returns details about that element.

Bug: 907380
Change-Id: I0478d125e2b3d1c44f4c5cea03d110b642424955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2002045Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731895}

ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js

@@ -286,6 +286,37 @@ test.util.sync.getActiveElement = (contentWindow, opt_styleNames) => {
       contentWindow.document.activeElement, contentWindow, opt_styleNames);
 };
 
+/**
+ * Gets the information of the active element. However, unlike the previous
+ * helper, the shadow roots are searched as well.
+ *
+ * @param {Window} contentWindow Window to be tested.
+ * @param {Array<string>=} opt_styleNames List of CSS property name to be
+ *     obtained.
+ * @return {?{attributes:Object<string>, text:string,
+ *                  styles:(Object<string>|undefined), hidden:boolean}} Element
+ *     information that contains contentText, attribute names and
+ *     values, hidden attribute, and style names and values. If there is no
+ *     active element, returns null.
+ */
+test.util.sync.findActiveElement = (contentWindow, opt_styleNames) => {
+  if (!contentWindow.document || !contentWindow.document.activeElement) {
+    return null;
+  }
+
+  let activeElement = contentWindow.document.activeElement;
+  while (true) {
+    const shadow = activeElement.shadowRoot;
+    if (shadow && shadow.activeElement) {
+      activeElement = shadow.activeElement;
+    } else {
+      break;
+    }
+  }
+
+  return extractElementInfo(activeElement, contentWindow, opt_styleNames);
+};
+
 /**
  * Assigns the text to the input element.
  * @param {Window} contentWindow Window to be tested.