[IndexedDB] Add request property to IDBCursor

https://w3c.github.io/IndexedDB/#dom-idbcursor-request

Chrome status:
https://chromestatus.com/feature/5432744081358848

Intent to Implement and Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/o6mUiI09BUc/25khjTHYAAAJ

Bug: 943344
Change-Id: I9c7c801ae266c0dd75cd9bc21355913865ecbcc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585892
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656396}

IndexedDB/idbcursor-request.any.js

@@ -18,6 +18,7 @@ function cursorRequestTest({ useIndex, useKeyCursor }) {
         cursor = req.result;
         assert_equals(cursor.request, req, 'cursor.request');
         assert_readonly(cursor, 'request');
+        assert_equals(cursor.request, cursor.request, 'cursor.request does not change');
       });
 
       req.transaction.oncomplete = t.step_func(() => {