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}
AnimationClock& Document::animationClock()
{
    ASSERT(page());
    return page()->animator().clock();
}

AnimationClock& Document::animationClock()
{
    ASSERT(page());
    return page()->animator().clock();
}
