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::unscheduleUseShadowTreeUpdate(SVGUseElement& element)
{
    m_useElementsNeedingUpdate.remove(&element);
}

void Document::unscheduleUseShadowTreeUpdate(SVGUseElement& element)
{
    m_useElementsNeedingUpdate.remove(&element);
}
