Bug 1853765 - Use BrowserTestUtils.is_visible instead of workaround in browser_file_migration.js. r=kpatenio

Differential Revision: https://phabricator.services.mozilla.com/D188517

browser/components/migration/tests/browser/browser_file_migration.js

@@ -291,14 +291,8 @@ add_task(async function test_file_migration_error() {
     );
 
     let errorMessageContainer = shadow.querySelector(".file-import-error");
-
-    // Using BrowserTestUtils.is_visible to check for the visibility of the
-    // message seems to throw as it works its way up the ancestry and hits
-    // the shadowRoot. We'll work around this by making sure that the
-    // boundingClientRect has a width and height.
-    let errorRect = errorMessageContainer.getBoundingClientRect();
     Assert.ok(
-      errorRect.width && errorRect.height,
+      BrowserTestUtils.is_visible(errorMessageContainer),
       "Should be showing the error message container"
     );