Add missing optional arguments to `Path2D` constructor in extern definition.
@@ -258,10 +258,11 @@ CanvasPathMethods.prototype.arc = function(
/**
* @constructor
+ * @param {!Path2D|string=} arg
* @implements {CanvasPathMethods}
* @see https://html.spec.whatwg.org/multipage/scripting.html#path2d-objects
*/
-function Path2D() {}
+function Path2D(arg) {}
/**
* @return {undefined}
