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}
bool Document::hasAppCacheManifest() const
{
    return isHTMLHtmlElement(documentElement()) && documentElement()->hasAttribute(manifestAttr);
}

bool Document::hasAppCacheManifest() const
{
    return isHTMLHtmlElement(documentElement()) && documentElement()->hasAttribute(manifestAttr);
}
