Add UseCounter entry for SVGSVGElementInDocument.

SVG elements not in the document may be skewing our SVG UseCounter
numbers. This patch adds a new counter entry, SVGSVGElementInDocument,
which will not trigger on SVG elements not in the document.

I've uploaded a few testcases at http://pr.gg/svgcount and manually
verified this usecounter entry does not fire on the Modernizr case
but does fire in: inline, standalone, and image contexts.

I'm leaving in the SVGSVGElement UseCounter to see how our numbers are
skewed by Modernizr and similar scripts. We also try to not change
existing UseCounter entries so that we have stable historical data.

Review URL: https://codereview.chromium.org/149833002

git-svn-id: svn://svn.chromium.org/blink/trunk@166070 bbb929c8-8fbe-4397-9dbb-9b2b20218538