Add toSVGFontFaceUri|NameElement(), and use them.

These functions have type checking assertions for helping to find bad-cast bugs.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23563005

git-svn-id: svn://svn.chromium.org/blink/trunk@157864 bbb929c8-8fbe-4397-9dbb-9b2b20218538