ElementShadow::addShadowRoot should use lazyReattachIfAttached

The if statement is identical to calling lazyReattachIfAttached.

Review URL: https://chromiumcodereview.appspot.com/23560004

git-svn-id: svn://svn.chromium.org/blink/trunk@156823 bbb929c8-8fbe-4397-9dbb-9b2b20218538