Change Document::detach() to RELEASE_ASSERT all subframes are gone.

BUG=556724,577105

Review URL: https://codereview.chromium.org/1667573002

Cr-Commit-Position: refs/heads/master@{#373642}
HTMLHeadElement* Document::head() const
{
    Node* de = documentElement();
    if (!de)
        return 0;

    return Traversal<HTMLHeadElement>::firstChild(*de);
}

HTMLHeadElement* Document::head() const
{
    Node* de = documentElement();
    if (!de)
        return 0;

    return Traversal<HTMLHeadElement>::firstChild(*de);
}
