Implement document.open and document.close (fixes #14591)

components/script/dom/bindings/str.rs

@@ -170,11 +170,16 @@ impl DOMString {
         self.0.push_str(string)
     }
 
-    /// Truncates this `DOMString`, removing all contents.
+    /// Clears this `DOMString`, removing all contents.
     pub fn clear(&mut self) {
         self.0.clear()
     }
 
+    /// Shortens this String to the specified length.
+    pub fn truncate(&mut self, new_len: usize) {
+        self.0.truncate(new_len);
+    }
+
     /// An iterator over the bytes of this `DOMString`.
     pub fn bytes(&self) -> Bytes {
         self.0.bytes()