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}
void Document::initSecurityContext()
{
    initSecurityContext(DocumentInit(m_url, m_frame, contextDocument(), m_importsController));
}

void Document::initSecurityContext()
{
    initSecurityContext(DocumentInit(m_url, m_frame, contextDocument(), m_importsController));
}
