[SVG] Devirtualize RenderSVGResource::resourceBoundingBox()

Only RenderSVGResourceFilter, RenderSVGResourceMasker and
RenderSVGResourceClipper have meaningful overrides, and all clients have
access to the qualified type.

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

Review URL: https://codereview.chromium.org/34463008

git-svn-id: svn://svn.chromium.org/blink/trunk@160388 bbb929c8-8fbe-4397-9dbb-9b2b20218538