Fix #655 - Allow createPattern to return null

source

@@ -59921,7 +59921,7 @@ interface <dfn>CanvasFillStrokeStyles</dfn> {
   attribute (DOMString or CanvasGradient or CanvasPattern) <span data-x="dom-context-2d-fillStyle">fillStyle</span>; // (default black)
   <span>CanvasGradient</span> <span data-x="dom-context-2d-createLinearGradient">createLinearGradient</span>(double x0, double y0, double x1, double y1);
   <span>CanvasGradient</span> <span data-x="dom-context-2d-createRadialGradient">createRadialGradient</span>(double x0, double y0, double r0, double x1, double y1, double r1);
-  <span>CanvasPattern</span> <span data-x="dom-context-2d-createPattern">createPattern</span>(<span>CanvasImageSource</span> image, [TreatNullAs=EmptyString] DOMString repetition);
+  <span>CanvasPattern?</span> <span data-x="dom-context-2d-createPattern">createPattern</span>(<span>CanvasImageSource</span> image, [TreatNullAs=EmptyString] DOMString repetition);
 <!--
   // v8 we received one request from Ralf Richard G&oml;bel for a new kind of pattern: a hatch.
   // basically it would be a series of dash styles, angles, line widths, and offsets