Add UseCounter entry for SVGForeignObjectElement

There have been discussions about removing foreignObject and instead
allow an html element with position and dimensions inside svg. Knowing
how popular foreignObject is may be useful in such discussions.

Review URL: https://codereview.chromium.org/220713003

git-svn-id: svn://svn.chromium.org/blink/trunk@170561 bbb929c8-8fbe-4397-9dbb-9b2b20218538