Add UseCounter for SVGLocatable.{nearest,farthest}ViewportElement

BUG=
R=pdr,schenney

Review URL: https://codereview.chromium.org/98573003

git-svn-id: svn://svn.chromium.org/blink/trunk@163005 bbb929c8-8fbe-4397-9dbb-9b2b20218538