Replace InsertionPoint::insertionPointType with tagName checks

insertionPointType() don't make sense since the only kind of insertion points are <content>
and <shadow> so we can replace it with a hasTagName check. This also means we don't need
to make virtual calls to make decisions during distribution.

Review URL: https://chromiumcodereview.appspot.com/19726010

git-svn-id: svn://svn.chromium.org/blink/trunk@154857 bbb929c8-8fbe-4397-9dbb-9b2b20218538