Port Node's previousElementSibling() / nextElementSibling() to ElementTraversal

Port Node's previousElementSibling() / nextElementSibling() to ElementTraversal
and update callers on native side to use ElementTraversal instead for
consistency.

R=adamk, eseidel

Review URL: https://codereview.chromium.org/166033005

git-svn-id: svn://svn.chromium.org/blink/trunk@167229 bbb929c8-8fbe-4397-9dbb-9b2b20218538