Const-ify static s_resourceType members in RenderSVGResouce* classes

This is a merge of http://trac.webkit.org/changeset/175117 by zandobersek@gmail.com

BUG=425730

Review URL: https://codereview.chromium.org/694283002

git-svn-id: svn://svn.chromium.org/blink/trunk@184819 bbb929c8-8fbe-4397-9dbb-9b2b20218538