CSS Animations: add interpolation test for width and height

Note: added <meta utf8> tag so these tests run in recent Firefoxen
without complaint.

Review URL: https://chromiumcodereview.appspot.com/23621020

git-svn-id: svn://svn.chromium.org/blink/trunk@157119 bbb929c8-8fbe-4397-9dbb-9b2b20218538