IndexedDB: Result of failed request should be |undefined|

Per spec, after an error the |result| should always be |undefined|. Chromium was yielding |null| or an obsolete value instead.

BUG=378411

Review URL: https://codereview.chromium.org/307653004

git-svn-id: svn://svn.chromium.org/blink/trunk@174997 bbb929c8-8fbe-4397-9dbb-9b2b20218538