Make CSSStyleImageValue a member of CanvasImageSource.

third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.idl

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 // http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#canvasrenderingcontext2d
 
-typedef (HTMLImageElement or
+typedef (CSSImageValue or
+         HTMLImageElement or
          HTMLVideoElement or
          HTMLCanvasElement or
          ImageBitmap or
          OffscreenCanvas) CanvasImageSource;
 
 enum CanvasFillRule { "nonzero", "evenodd" };
 enum ImageSmoothingQuality {"low", "medium", "high"};
 
 [
     SetWrapperReferenceFrom=canvas,
@@ skipping 101 matching lines @@
 
     // text
     attribute DOMString font; // (default 10px sans-serif)
     attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start")
     attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
     [RuntimeEnabled=ExperimentalCanvasFeatures] attribute DOMString direction; // "inherit", "rtl", "ltr" (default: "inherit")
 
 };
 
 CanvasRenderingContext2D implements CanvasPathMethods;