[whatwg] Specify getSVGDocument

[Philip Jägenstedt]

http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=2643

In my testing, getSVGDocument is supported on <embed>, <frame>,
<iframe> and <object> in Firefox Nightly, IE11 Preview, Safari 7.0 and
Opera 12.16, the only exception being frame.getSVGDocument in Firefox.

I don't know if this API is actually useful, but since it's already
implemented everywhere it should probably be spec'd.

Philip