[IndexedDB]: Add commit check to aborting txns.

A check is added to ensure that transactions that have already been sent
a commit signal from the front end (for whatever reason) are committed
instead of aborted in the event of an abort.

Spec Change: w3c/IndexedDB#242

Explainer: https://andreas-butler.github.io/idb-transaction-commit/EXPLAINER
Change-Id: I942136c1bbfb8a5347bf7cf1f9702f5cbff66521
Reviewed-on: https://chromium-review.googlesource.com/c/1330692
Commit-Queue: Andreas Butler <andreasbutler@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619859}

IndexedDB/keypath-special-identifiers.htm

@@ -54,6 +54,8 @@
         const result = request.result;
         assert_key_equals(result[testcase.property], key,
                           'Property should be used as key');
+      });
+      tx.oncomplete = t.step_func(function() {
         t.done();
       });
     },