Auto merge of #21052 - tigercosmos:rrr, r=jdm

add TextLength

<!-- Please describe your changes on the following line: -->
add TextLength in HTMLTextAreaElement

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- [ ] These changes fix #__ (github issue number if applicable).

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components/script/dom/htmltextareaelement.rs

@@ -263,6 +263,11 @@ impl HTMLTextAreaElementMethods for HTMLTextAreaElement {
         self.upcast::<Node>().dirty(NodeDamage::OtherNodeDamage);
     }
 
+    // https://html.spec.whatwg.org/multipage/#dom-textarea-textlength
+    fn TextLength(&self) -> u32 {
+       self.textinput.borrow().utf16_len() as u32
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-lfe-labels
     fn Labels(&self) -> DomRoot<NodeList> {
         self.upcast::<HTMLElement>().labels()

components/script/dom/webidls/HTMLTextAreaElement.webidl

@@ -39,7 +39,7 @@ interface HTMLTextAreaElement : HTMLElement {
            attribute DOMString defaultValue;
   [CEReactions,TreatNullAs=EmptyString]
            attribute DOMString value;
-  // readonly attribute unsigned long textLength;
+  readonly attribute unsigned long textLength;
 
   // readonly attribute boolean willValidate;
   // readonly attribute ValidityState validity;