Add lint check for &DomRoot<T>

`&DomRoot<T> is strictly less expressive than `&T`, so using it is
pointless.

python/tidy/servo_tidy/tidy.py

@@ -623,6 +623,7 @@ def check_rust(file_name, lines):
             (r"DomRefCell<Heap<.+>>", "Banned type DomRefCell<Heap<T>> detected. Use MutDom<T> instead", no_filter),
             # No benefit to using &Root<T>
             (r": &Root<", "use &T instead of &Root<T>", no_filter),
+            (r": &DomRoot<", "use &T instead of &DomRoot<T>", no_filter),
             (r"^&&", "operators should go at the end of the first line", no_filter),
             # This particular pattern is not reentrant-safe in script_thread.rs
             (r"match self.documents.borrow", "use a separate variable for the match expression",

python/tidy/servo_tidy_tests/rust_tidy.rs

@@ -38,7 +38,7 @@ impl test {
         }
     }
 
-    fn test_fun2(y : &String, z : &Vec<f32>, r: &Root<isize>) -> () {
+    fn test_fun2(y : &String, z : &Vec<f32>, r: &Root<isize>, s: &DomRoot<isize>) -> () {
         let x = true;
         x
             && x;

python/tidy/servo_tidy_tests/test_tidy.py

@@ -112,6 +112,7 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('use &[T] instead of &Vec<T>', next(errors)[2])
         self.assertEqual('use &str instead of &String', next(errors)[2])
         self.assertEqual('use &T instead of &Root<T>', next(errors)[2])
+        self.assertEqual('use &T instead of &DomRoot<T>', next(errors)[2])
         self.assertEqual('encountered function signature with -> ()', next(errors)[2])
         self.assertEqual('operators should go at the end of the first line', next(errors)[2])
         self.assertNoMoreErrors(errors)