Update interfaces/SVG.idl (#14746)

Source: https://github.com/tidoust/reffy-reports/blob/0b25b9f/whatwg/idl/SVG.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/477693484
diff --git a/interfaces/SVG.idl b/interfaces/SVG.idl
index 9588ad0..78b9d64 100644
--- a/interfaces/SVG.idl
+++ b/interfaces/SVG.idl
@@ -262,7 +262,7 @@
   DOMMatrix createSVGMatrix();
   DOMRect createSVGRect();
   SVGTransform createSVGTransform();
-  SVGTransform createSVGTransformFromMatrix(DOMMatrixReadOnly matrix);
+  SVGTransform createSVGTransformFromMatrix(optional DOMMatrix2DInit matrix);
 
   Element getElementById(DOMString elementId);
 
@@ -367,7 +367,7 @@
   [SameObject] readonly attribute DOMMatrix matrix;
   readonly attribute float angle;
 
-  void setMatrix(DOMMatrixReadOnly matrix);
+  void setMatrix(optional DOMMatrix2DInit matrix);
   void setTranslate(float tx, float ty);
   void setScale(float sx, float sy);
   void setRotate(float angle, float cx, float cy);
@@ -391,7 +391,7 @@
   setter void (unsigned long index, SVGTransform newItem);
 
   // Additional methods not common to other list interfaces.
-  SVGTransform createSVGTransformFromMatrix(DOMMatrixReadOnly matrix);
+  SVGTransform createSVGTransformFromMatrix(optional DOMMatrix2DInit matrix);
   SVGTransform? consolidate();
 };