Bug 1125562 - Mark some const attributes in webidl as such r=bzbarsky

dom/webidl/SVGSVGElement.webidl

@@ -19,9 +19,13 @@ interface SVGSVGElement : SVGGraphicsElement {
   readonly attribute SVGAnimatedLength width;
   readonly attribute SVGAnimatedLength height;
   // readonly attribute SVGRect viewport;
+  [Constant]
   readonly attribute float pixelUnitToMillimeterX;
+  [Constant]
   readonly attribute float pixelUnitToMillimeterY;
+  [Constant]
   readonly attribute float screenPixelToMillimeterX;
+  [Constant]
   readonly attribute float screenPixelToMillimeterY;
   readonly attribute boolean useCurrentView;
   // readonly attribute SVGViewSpec currentView;