Reset algorithm is incorrect for select element with first option disabled

As per the spec (https://www.w3.org/TR/html5/forms.html#ask-for-a-reset),
the user agent must set the selectedness of the first option element in the list
of options in tree order that is not disabled, if any, to true.

BUG=578705

Review URL: https://codereview.chromium.org/1604653005

Cr-Commit-Position: refs/heads/master@{#370303}
2 files changed