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}
PassRefPtrWillBeRawPtr<Text> Document::createTextNode(const String& data)
{
    return Text::create(*this, data);
}

PassRefPtrWillBeRawPtr<Text> Document::createTextNode(const String& data)
{
    return Text::create(*this, data);
}
