Move highestAncestorOrSelf() from Node to NodeTraversal

Move highestAncestorOrSelf() from Node to NodeTraversal as this is a DOM
traversal utility function.

This CL also simplifies the method implementation.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/425383002

git-svn-id: svn://svn.chromium.org/blink/trunk@179263 bbb929c8-8fbe-4397-9dbb-9b2b20218538