Auto merge of #9109 - nerith:createpattern, r=jdm

Support empty strings as the repeat argument (CreatePattern)

According to the third step in the specification [1], createPattern
should let the repetition argument be "repeat" when it is the empty
string.

The code in CanvasRenderingContext2D::CreatePattern did not implement
this step and instead threw a SyntaxError exception when an empty
string was supplied as the repetition argument.

Fixes #9079.

[1] https://html.spec.whatwg.org/multipage/#dom-context-2d-createpattern

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components/script/dom/canvasrenderingcontext2d.rs

@@ -1154,7 +1154,7 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
     // https://html.spec.whatwg.org/multipage/#dom-context-2d-createpattern
     fn CreatePattern(&self,
                      image: HTMLImageElementOrHTMLCanvasElementOrCanvasRenderingContext2D,
-                     repetition: DOMString)
+                     mut repetition: DOMString)
                      -> Fallible<Root<CanvasPattern>> {
         let (image_data, image_size) = match image {
             HTMLImageElementOrHTMLCanvasElementOrCanvasRenderingContext2D::eHTMLImageElement(ref image) => {
@@ -1176,6 +1176,10 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
             }
         };
 
+        if repetition.is_empty() {
+            repetition.push_str("repeat");
+        }
+
         if let Ok(rep) = RepetitionStyle::from_str(&repetition) {
             Ok(CanvasPattern::new(self.global.root().r(),
                                   image_data,