Throw when resizing OffscreenCanvas placeholder

Attempting to set the width or height attribute of a canvas element
after it has transferred control to an OffscreenCanvas should throw an
exception. Before this change, the behavior was to silently ignore
changes to these attributes, which was not clear because that case was
not specifically mentioned in the spec.

source

@@ -59196,6 +59196,13 @@ callback <dfn>BlobCallback</dfn> = void (<span>Blob</span>? blob);</pre>
   attribute defaults to 300, and the <code data-x="attr-canvas-height">height</code> attribute
   defaults to 150.</p>
 
+  <p>When setting the value of the <code data-x="attr-canvas-width">width</code> or <code
+  data-x="attr-canvas-height">height</code> attribute, if the <span
+  data-x="concept-canvas-context-mode">context mode</span> of the <code>canvas</code>
+  element is set to <span data-x="concept-canvas-placeholder">placeholder</span>, the
+  user agent must throw an <span>"<code>InvalidStateError</code>"</span> <code>DOMException</code>
+  and leave the attribute's value unchanged.</p>
+
   <p>The <span>intrinsic dimensions</span> of the <code>canvas</code> element when it
   <span>represents</span> <span>embedded content</span> are equal to the dimensions of the
   element's bitmap.</p>