Remove unnecessary Shadow DOM encapsulation checks

NodeRenderingContext had methods on it to check for "upper encaupsulation boundary"
and "lower encapsulation boundary" that things were using to try to control
renderer creation but this is wrong, they should be using insertion points instead.
It turns out these checks didn't do anything anyway since having a ShadowRoot with
no InsertionPoint means all your children don't render.

It also turns out that InsertionPoint::isShadowBoundary is identical to isActive.

R=morrita@chromium.org

Review URL: https://codereview.chromium.org/20055003

git-svn-id: svn://svn.chromium.org/blink/trunk@154815 bbb929c8-8fbe-4397-9dbb-9b2b20218538