Move Node/ContainerNode's traverseToChildAt() to NodeTraversal

Move Node/ContainerNode's traverseToChildAt() to NodeTraversal and rename to
childAt() as this method does DOM Tree traversal and we already have a utility
class for this.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/425223005

git-svn-id: svn://svn.chromium.org/blink/trunk@179358 bbb929c8-8fbe-4397-9dbb-9b2b20218538