Add toSVGGlyphRefElement(), and use it as much as possible

This function will help to cast with type checking assertions in order to detect
bad-cast bugs.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/23766031

git-svn-id: svn://svn.chromium.org/blink/trunk@157729 bbb929c8-8fbe-4397-9dbb-9b2b20218538