Use new Traversal<> API more in the code base

Use new Traversal<> API more in the code base to simplify code a bit and
increase readability.

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/420193007

git-svn-id: svn://svn.chromium.org/blink/trunk@179067 bbb929c8-8fbe-4397-9dbb-9b2b20218538