Improve XML document UseCounters

This patch makes the XMLDocument UseCounter useful by ignoring cases
where a frameView is not available (e.g., responseXML). This patch
also introduces SVGSVGElementInXMLDocument which counts both
SVG-as-image and SVG top-level documents, allowing us to separate
SVG XML usage from the total XMLDocument count.

Tested cases:
ajax request with responseXML[1] - XMLDocument no longer counts this
xml doc as root[2] - XMLDocument(+1)
svg doc as root[3] - XMLDocument(+1), SVGSVGElementInXMLDocument(+1)
svg doc in image[4] - XMLDocument(+1), SVGSVGElementInXMLDocument(+1)
svg inline[5] - Neither XMLDocument nor SVGSVGElementInXMLDocument

[1] http://www.w3schools.com/ajax/ajax_xmlfile.asp
[2] http://www.w3.org/Style/Examples/001/doc.xml
[3] http://pr.gg/svg.svg
[4] http://pr.gg/svg.html
[5] http://pr.gg/inline.html

BUG=361411

Review URL: https://codereview.chromium.org/235883004

git-svn-id: svn://svn.chromium.org/blink/trunk@171704 bbb929c8-8fbe-4397-9dbb-9b2b20218538