[SVG] Const-ify resourceBoundingBox()'s argument

Minor cleanup, to remove an unnecessary const-cast in
SVGRenderSupport::intersectRepaintRectWithResources().

R=schenney@chromium.org,pdr@chromium.org

Review URL: https://codereview.chromium.org/143873013

git-svn-id: svn://svn.chromium.org/blink/trunk@165571 bbb929c8-8fbe-4397-9dbb-9b2b20218538