Replace SVGDocument by XMLDocument since it is removed from SVG2. In order to achieve this move its functionality to SVGDocumentExtensions and adjust the call sites. XMLDocument now knows how to create a SVG Document and getSVGDocument returns a Document. Finally mark XMLDocument as final.

Review URL: https://codereview.chromium.org/268803005

git-svn-id: svn://svn.chromium.org/blink/trunk@173338 bbb929c8-8fbe-4397-9dbb-9b2b20218538