Have ElementTraversal::firstWithin() take a reference

Have ElementTraversal::firstWithin() take a reference instead of a pointer
as its implementation expects the pointer to be non-null. This makes the
code look safer.

R=adamk

Review URL: https://codereview.chromium.org/68173014

git-svn-id: svn://svn.chromium.org/blink/trunk@161969 bbb929c8-8fbe-4397-9dbb-9b2b20218538