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}
Element* Document::pointerLockElement() const
{
    if (!page() || page()->pointerLockController().lockPending())
        return 0;
    if (Element* element = page()->pointerLockController().element()) {
        if (element->document() == this)
            return element;
    }
    return 0;
}

Element* Document::pointerLockElement() const
{
    if (!page() || page()->pointerLockController().lockPending())
        return 0;
    if (Element* element = page()->pointerLockController().element()) {
        if (element->document() == this)
            return element;
    }
    return 0;
}
