[Shadow DOM]: Empty shadow insertion points should behave like <shadow><content *leftover*></content></shadow>

This is a follow up patch for r161137.
We've changed the spec so that we keep a backward compatibility for empty shadow insertion points.

The spec side issue is https://www.w3.org/Bugs/Public/show_bug.cgi?id=23767

BUG=316513

Review URL: https://codereview.chromium.org/59903015

git-svn-id: svn://svn.chromium.org/blink/trunk@161702 bbb929c8-8fbe-4397-9dbb-9b2b20218538