Update expected result of an unconfigured canvas to be transparent black (#4607)

Author: teoxoy

src/webgpu/web_platform/canvas/readbackFromWebGPUCanvas.spec.ts

@@ -519,14 +519,8 @@ g.test('transferToImageBitmap_unconfigured_nonzero_size')
       return;
     }
 
-    // Since there isn't a configuration we expect the ImageBitmap to have the default alphaMode of "opaque".
+    // Since there isn't a configuration we expect the ImageBitmap to be transparent black.
     const expected = new Uint8ClampedArray(kWidth * kHeight * 4);
-    for (let i = 0; i < expected.byteLength; i += 4) {
-      expected[i + 0] = 0;
-      expected[i + 1] = 0;
-      expected[i + 2] = 0;
-      expected[i + 3] = 255;
-    }
 
     readbackContext.drawImage(ib, 0, 0);
     readPixelsFrom2DCanvasAndCompare(t, readbackContext, expected);