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::removeFocusedElementOfSubtree(Node* node, bool amongChildrenOnly)
{
    if (!m_focusedElement)
        return;

    if (!node->inDocument())
        return;
    bool contains = node->containsIncludingShadowDOM(m_focusedElement.get());
    if (contains && (m_focusedElement != node || !amongChildrenOnly))
        clearFocusedElement();
}

void Document::removeFocusedElementOfSubtree(Node* node, bool amongChildrenOnly)
{
    if (!m_focusedElement)
        return;

    if (!node->inDocument())
        return;
    bool contains = node->containsIncludingShadowDOM(m_focusedElement.get());
    if (contains && (m_focusedElement != node || !amongChildrenOnly))
        clearFocusedElement();
}
