Revert 161702 "[Shadow DOM]: Empty shadow insertion points shoul..."

This reverts commit 1d30145f59d2316df295abec1001e3d49f7c555c.

> [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

TBR=hayato@chromium.org
BUG=318057

Review URL: https://codereview.chromium.org/70573003

git-svn-id: svn://svn.chromium.org/blink/trunk@161914 bbb929c8-8fbe-4397-9dbb-9b2b20218538