Experimental SVG 1.1 DOM runtime flag.

This adds UseCounters for SVG DOM, to see how much usage there is.

SVG DOM support is enabled by default, but it will be possible to disable it with a commandline flag (--disable-svg1dom), see https://codereview.chromium.org/577943002.

BUG=415074

Review URL: https://codereview.chromium.org/474013004

git-svn-id: svn://svn.chromium.org/blink/trunk@183898 bbb929c8-8fbe-4397-9dbb-9b2b20218538