Either content elements or shadow elements should not be treated as insertion points if they are not in document.

The relevant spec update: https://github.com/w3c/webcomponents/commit/69fbceabe01786ba3740728a6b86f5ec958f69b4

This behavior is similar to that of styles, which are not calculated for disconnected nodes.

This change also makes it possible to add an assertion, ASSERT(shadowRoot), for rootNode of an insertionPoint in InsertionPoint::isActive.

BUG=339029

Review URL: https://codereview.chromium.org/133213008

git-svn-id: svn://svn.chromium.org/blink/trunk@166647 bbb929c8-8fbe-4397-9dbb-9b2b20218538