Add toSVGImage fuction, and use it as much as we can.

It has type checking assertions, and also it will help to find bad-cast bugs.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24071003

git-svn-id: svn://svn.chromium.org/blink/trunk@157628 bbb929c8-8fbe-4397-9dbb-9b2b20218538