The LayoutTest results for LayoutTests/svg/custom/feColorMatrix-value.svg differs bewteen the Gtk bots (http://build.webkit.org/waterfall). The difference is only 1px in width and height for the RenderSVGContainers. Can't provide the differences here on this bug. The bots were broken during opening this bug report.
Added the test to the gtk-skip-list: http://trac.webkit.org/changeset/57007
Changed component to SVG, so it shows up in my all-svg-bugs search.
This doesn't seem to be an issue any longer.