Have Node::lastDescendant() return a reference

      Have Node::lastDescendant() return a reference instead of a pointer
as it can never return null. It makes the calling code look safer
and gets rid of several unnecessary null checks.

Also get rid of Node::firstDescendant() which is unused.

R=adamk

Review URL: https://codereview.chromium.org/54803003

git-svn-id: svn://svn.chromium.org/blink/trunk@161096 bbb929c8-8fbe-4397-9dbb-9b2b20218538