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::setParsingState(ParsingState parsingState)
{
    m_parsingState = parsingState;

    if (parsing() && !m_elementDataCache)
        m_elementDataCache = ElementDataCache::create();
}

void Document::setParsingState(ParsingState parsingState)
{
    m_parsingState = parsingState;

    if (parsing() && !m_elementDataCache)
        m_elementDataCache = ElementDataCache::create();
}
