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}
AtomicString Document::convertLocalName(const AtomicString& name)
{
    return isHTMLDocument() ? name.lower() : name;
}

AtomicString Document::convertLocalName(const AtomicString& name)
{
    return isHTMLDocument() ? name.lower() : name;
}
