Regression(r178540): Traversal<*Element>::lastWithin() is incorrect

Traversal<*Element>::lastWithin() is incorrect after r178540 because it
changed from using ElementTraversal::lastWithin() to using Node.lastChild()
when doing node traversal. The proper API for this is
NodeTraversal::lastWithin() as we want the last descendant, not the last
child.

This was not caught by layout tests because this template specialization
is currently unused. Current callers are using the simpler Traversal<Element>
template specialization.

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

Review URL: https://codereview.chromium.org/416933002

git-svn-id: svn://svn.chromium.org/blink/trunk@179057 bbb929c8-8fbe-4397-9dbb-9b2b20218538