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}
DocumentState* Document::formElementsState() const
{
    if (!m_formController)
        return 0;
    return m_formController->formElementsState();
}

DocumentState* Document::formElementsState() const
{
    if (!m_formController)
        return 0;
    return m_formController->formElementsState();
}
