Remove null checks and extra isElementNode() checks from ShadowDOMSiblingTraversalStrategy

ShadowDOMSiblingTraversalStrategy was checking for null siblings which is impossible
and was also checking for isElementNode() which was redundant since the following
hasTagName calls contained that check already.

Review URL: https://chromiumcodereview.appspot.com/19734004

git-svn-id: svn://svn.chromium.org/blink/trunk@154670 bbb929c8-8fbe-4397-9dbb-9b2b20218538