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::setImportsController(HTMLImportsController* controller)
{
    ASSERT(!m_importsController || !controller);
    m_importsController = controller;
    if (!m_importsController && !loader())
        m_fetcher->clearContext();
}

void Document::setImportsController(HTMLImportsController* controller)
{
    ASSERT(!m_importsController || !controller);
    m_importsController = controller;
    if (!m_importsController && !loader())
        m_fetcher->clearContext();
}
