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::clearFocusedElementSoon()
{
    if (!m_clearFocusedElementTimer.isActive())
        m_clearFocusedElementTimer.startOneShot(0, BLINK_FROM_HERE);
}

void Document::clearFocusedElementSoon()
{
    if (!m_clearFocusedElementTimer.isActive())
        m_clearFocusedElementTimer.startOneShot(0, BLINK_FROM_HERE);
}
