Use `const` instead of `static mut` in script/dom/eventtarget.rs

There is no mutability there.

components/script/dom/eventtarget.rs

@@ -515,22 +515,16 @@ impl EventTarget {
         let name = CString::new(&**ty).unwrap();
 
         // Step 3.9, subsection ParameterList
-        static mut ARG_NAMES: [*const c_char; 1] = [b"event\0" as *const u8 as *const c_char];
-        static mut ERROR_ARG_NAMES: [*const c_char; 5] = [
+        const ARG_NAMES: &[*const c_char] = &[b"event\0" as *const u8 as *const c_char];
+        const ERROR_ARG_NAMES: &[*const c_char] = &[
             b"event\0" as *const u8 as *const c_char,
             b"source\0" as *const u8 as *const c_char,
             b"lineno\0" as *const u8 as *const c_char,
             b"colno\0" as *const u8 as *const c_char,
             b"error\0" as *const u8 as *const c_char,
         ];
         let is_error = ty == &atom!("error") && self.is::<Window>();
-        let args = unsafe {
-            if is_error {
-                &ERROR_ARG_NAMES[..]
-            } else {
-                &ARG_NAMES[..]
-            }
-        };
+        let args = if is_error { ERROR_ARG_NAMES } else { ARG_NAMES };
 
         let cx = window.get_cx();
         let options = unsafe {