Have NodeTraversal::previous*() take a reference

Have NodeTraversal::previous*() take a reference in argument instead of a
pointer as the implementation 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/69003004

git-svn-id: svn://svn.chromium.org/blink/trunk@161861 bbb929c8-8fbe-4397-9dbb-9b2b20218538