IndexedDB cursor.request (#15820)

* cursor.request

* More detailed test, testing the various different kinds of cursor.

* Using unreached_func

IndexedDB/idbcursor-request.any.js

@@ -0,0 +1,40 @@
+// META: script=support.js
+
+function cursorRequestTest({ useIndex, useKeyCursor }) {
+  indexeddb_test(
+    (t, db) => {
+      const objStore = db.createObjectStore("my_objectstore");
+      objStore.add("data",  1);
+      objStore.createIndex("my_index", "");
+    },
+    (t, db) => {
+      const tx = db.transaction("my_objectstore");
+      let source = tx.objectStore("my_objectstore");
+      if (useIndex) source = source.index('my_index');
+      const req = useKeyCursor ? source.openKeyCursor() : source.openCursor();
+      let cursor;
+
+      req.onsuccess = t.step_func(() => {
+        cursor = req.result;
+        assert_equals(cursor.request, req, 'cursor.request');
+        assert_readonly(cursor, 'request');
+      });
+
+      req.transaction.oncomplete = t.step_func(() => {
+        setTimeout(t.step_func(() => {
+          assert_equals(cursor.request, req, 'cursor.request after transaction complete');
+          t.done();
+        }), 0);
+      });
+
+      req.transaction.onerror = t.unreached_func('Transaction error');
+    },
+    `cursor.request from ${useIndex ? 'IDBIndex' : 'IDBObjectStore'}.${useKeyCursor ? 'openKeyCursor' : 'openCursor'}`
+  );
+}
+
+for (const useIndex of [false, true]) {
+  for (const useKeyCursor of [false, true]) {
+    cursorRequestTest({ useIndex, useKeyCursor });
+  }
+}