SELECT element: Fix a DCHECK failure after crrev.com/404091

On Android, selectedOption(), which is the first selected OPTION in tree order,
and m_lastOnChangeOption can be different because Android has menulist+multiple
configuration.

BUG=626328
TBR=keishi@chromium.org

Review-Url: https://codereview.chromium.org/2129023002
Cr-Commit-Position: refs/heads/master@{#404302}
1 file changed