[NFC] Cleanup video test (#4585)

* Inline visibleRect width and height access
* Assert visibleRect is non-null

Author: alan-baker

src/webgpu/web_platform/external_texture/video.spec.ts

@@ -406,9 +406,7 @@ parameters are present.
       // quadrant. In this test we crop the video to each quadrant and check that desired color
       // is sampled from each corner of the cropped image.
       // visible rect clip applies on raw decoded frame, which defines based on video frame visible size.
-      const visibleRect = source.visibleRect ?? new DOMRectReadOnly();
-      const srcVideoHeight = visibleRect.height;
-      const srcVideoWidth = visibleRect.width;
+      const visibleRect = source.visibleRect!;
 
       const srcColorSpace = kVideoInfo[videoName].colorSpace;
       const presentColors = kVideoExpectedColors[srcColorSpace][dstColorSpace];
@@ -423,38 +421,38 @@ parameters are present.
           subRect: {
             x: visibleRect.x,
             y: visibleRect.y,
-            width: srcVideoWidth / 2,
-            height: srcVideoHeight / 2,
+            width: visibleRect.width / 2,
+            height: visibleRect.height / 2,
           },
           color: convertToUnorm8(presentColors[expect.topLeftColor]),
         },
         // Top right
         {
           subRect: {
-            x: visibleRect.x + srcVideoWidth / 2,
+            x: visibleRect.x + visibleRect.width / 2,
             y: visibleRect.y,
-            width: srcVideoWidth / 2,
-            height: srcVideoHeight / 2,
+            width: visibleRect.width / 2,
+            height: visibleRect.height / 2,
           },
           color: convertToUnorm8(presentColors[expect.topRightColor]),
         },
         // Bottom left
         {
           subRect: {
             x: visibleRect.x,
-            y: visibleRect.y + srcVideoHeight / 2,
-            width: srcVideoWidth / 2,
-            height: srcVideoHeight / 2,
+            y: visibleRect.y + visibleRect.height / 2,
+            width: visibleRect.width / 2,
+            height: visibleRect.height / 2,
           },
           color: convertToUnorm8(presentColors[expect.bottomLeftColor]),
         },
         // Bottom right
         {
           subRect: {
-            x: visibleRect.x + srcVideoWidth / 2,
-            y: visibleRect.y + srcVideoHeight / 2,
-            width: srcVideoWidth / 2,
-            height: srcVideoHeight / 2,
+            x: visibleRect.x + visibleRect.width / 2,
+            y: visibleRect.y + visibleRect.height / 2,
+            width: visibleRect.width / 2,
+            height: visibleRect.height / 2,
           },
           color: convertToUnorm8(presentColors[expect.bottomRightColor]),
         },