Add getTransform() for CanvasRenderingContext2D (#1966)

Author: scholtzan

crates/web-sys/webidls/enabled/CanvasRenderingContext2D.webidl

@@ -146,6 +146,8 @@ interface mixin CanvasTransform {
   void setTransform(double a, double b, double c, double d, double e, double f);
   [Throws]
   void resetTransform();
+  [NewObject, Throws]
+  DOMMatrix getTransform();
 };
 
 [NoInterfaceObject]