Move Node::lastDescendantOrSelf() to NodeTraversal and rename to lastWithinOrSelf()

Move Node::lastDescendantOrSelf() to NodeTraversal and rename to
lastWithinOrSelf() as this is a node traversal utility function and it avoids
code duplication with NodeTraversal::lastWithin().

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

Review URL: https://codereview.chromium.org/420323002

git-svn-id: svn://svn.chromium.org/blink/trunk@179062 bbb929c8-8fbe-4397-9dbb-9b2b20218538