Handle delay between context invalidation and IDB request state.

Differential Revision: https://phabricator.services.mozilla.com/D203270

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1878146
gecko-commit: 0d93f14cf7ca90ca61ea3ecd31b6992446a4a77c
gecko-reviewers: asuth

IndexedDB/ready-state-destroyed-execution-context.html

@@ -21,6 +21,10 @@
     const openRequest = iframe.contentWindow.indexedDB.open(dbname);
     assert_equals(openRequest.readyState, 'pending');
     iframe.remove();
+    await new Promise(resolve => {
+      openRequest.onerror = resolve;
+      openRequest.onsuccess = resolve;
+    });
     assert_equals(openRequest.readyState, 'done');
 }, 'readyState accessor is valid after execution context is destroyed');