Have NodeTraversal::nextSkippingChildren() take a reference

Have NodeTraversal::nextSkippingChildren() take a reference instead of a pointer
as it expects the pointer to be non-null. This makes the code look safer and is
consistent with NodeTraversal::next().

R=adamk

Review URL: https://codereview.chromium.org/69543007

git-svn-id: svn://svn.chromium.org/blink/trunk@161935 bbb929c8-8fbe-4397-9dbb-9b2b20218538