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}
AXObjectCache* Document::existingAXObjectCache() const
{
    if (!axObjectCacheOwner().layoutView())
        return 0;

    return axObjectCacheOwner().m_axObjectCache.get();
}

AXObjectCache* Document::existingAXObjectCache() const
{
    if (!axObjectCacheOwner().layoutView())
        return 0;

    return axObjectCacheOwner().m_axObjectCache.get();
}
