Turn SVGRenderSupport into a namespace

SVGRenderSupport is not a class and it should not pretend to be one.
This patch converts it to a namespace.

Review URL: https://codereview.chromium.org/191293003

git-svn-id: svn://svn.chromium.org/blink/trunk@168838 bbb929c8-8fbe-4397-9dbb-9b2b20218538