Remove special case for ShadowRoot in notifyNodeInserted.

ShadowRoot doesn't really want to call notifyNodeInserted, it just
wants to call insertedInto directly. This also allows making the
notifyNodeInserted private.

I also made notifyNodeRemoved private and cleaned up
ElementShadow::addShadowRoot removing some ASSERTs that didn't make
sense since they just seemed to be checking if the linked list class
was implemented properly.

Review URL: https://codereview.chromium.org/425213002

git-svn-id: svn://svn.chromium.org/blink/trunk@179291 bbb929c8-8fbe-4397-9dbb-9b2b20218538