Add cloning steps for textarea

This makes textarea cloning work like input cloning, and matches Chrome.
Safari and Firefox have indicated they are willing to move in this
direction as well, as it is more developer-friendly for textarea to
behave the same as input. Closes #1233.

source

@@ -49562,6 +49562,11 @@ interface <dfn>HTMLTextAreaElement</dfn> : <span>HTMLElement</span> {
   interacts with the control in a way that changes the <span data-x="concept-textarea-raw-value">raw
   value</span>.</p>
 
+  <p>The <span data-x="concept-node-clone-ext">cloning steps</span> for <code>textarea</code>
+  elements must propagate the <span data-x="concept-textarea-raw-value">raw value</span> and <span
+  data-x="concept-textarea-dirty">dirty value flag</span> from the node being cloned to the
+  copy.</p>
+
   <p>When the <code>textarea</code> element's <code>textContent</code> IDL attribute changes value,
   if the element's <span data-x="concept-textarea-dirty">dirty value flag</span> is false, then the
   element's <span data-x="concept-textarea-raw-value">raw value</span> must be set to the value of