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}
PassRefPtrWillBeRawPtr<Document> Document::cloneDocumentWithoutChildren()
{
    DocumentInit init(url());
    if (isXMLDocument()) {
        if (isXHTMLDocument())
            return XMLDocument::createXHTML(init.withRegistrationContext(registrationContext()));
        return XMLDocument::create(init);
    }
    return create(init);
}

PassRefPtrWillBeRawPtr<Document> Document::cloneDocumentWithoutChildren()
{
    DocumentInit init(url());
    if (isXMLDocument()) {
        if (isXHTMLDocument())
            return XMLDocument::createXHTML(init.withRegistrationContext(registrationContext()));
        return XMLDocument::create(init);
    }
    return create(init);
}
