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::setContent(const String& content)
{
    open();
    m_parser->append(content);
    close();
}

void Document::setContent(const String& content)
{
    open();
    m_parser->append(content);
    close();
}
