Auto merge of #17950 - DarkDrek:downcast_to_is, r=jdm

Replace downcast with is

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I searched for all places where `if let Some(_) = self.downcast::` and replaced them with `::is`

---
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [X] These changes fix #17697 (github issue number if applicable).

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- [X] These changes do not require tests because `if let Some(_) = self.downcast::<Text>()` should be equivalent to `if self.is::<Text>()`

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components/script/dom/characterdata.rs

@@ -91,7 +91,7 @@ impl CharacterDataMethods for CharacterData {
         // If this is a Text node, we might need to re-parse (say, if our parent
         // is a <style> element.) We don't need to if this is a Comment or
         // ProcessingInstruction.
-        if let Some(_) = self.downcast::<Text>() {
+        if self.is::<Text>() {
             if let Some(parent_node) = node.GetParentNode() {
                 let mutation = ChildrenMutation::ChangeText;
                 vtable_for(&parent_node).children_changed(&mutation);

components/script/webdriver_handlers.rs

@@ -392,7 +392,7 @@ pub fn handle_is_selected(documents: &Documents,
             else if let Some(option_element) = node.downcast::<HTMLOptionElement>() {
                 Ok(option_element.Selected())
             }
-            else if let Some(_) = node.downcast::<HTMLElement>() {
+            else if node.is::<HTMLElement>() {
                 Ok(false) // regular elements are not selectable
             } else {
                 Err(())