[IndexedDB] Testing ordering and error handling for transaction.commit

* Tests that transaction ordering is consistent, even if a transaction
  calls commit().
* Tests that request errors cause a commit()-ed transaction to abort.

The second case seems to be broken in our implementation.

R=andreasbutler@google.com, cmp@chromium.org

Bug: 911877
Change-Id: I47da6ce9d350f6c47afcd02cf808d4fefb5f013f
Reviewed-on: https://chromium-review.googlesource.com/c/1362477
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Chase Phillips <cmp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614512}
3 files changed