Use a BTreeMap to store formdata

I'm really unsure about the MallocSizeOf of BTreeMap as I took the same
code as for HashMap.

Fixes #13105
Fixes #21381

components/malloc_size_of/lib.rs

@@ -557,6 +557,36 @@ where
     }
 }
 
+impl<K, V> MallocShallowSizeOf for std::collections::BTreeMap<K, V>
+where
+    K: Eq + Hash,
+{
+    fn shallow_size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        if ops.has_malloc_enclosing_size_of() {
+            self.values()
+                .next()
+                .map_or(0, |v| unsafe { ops.malloc_enclosing_size_of(v) })
+        } else {
+            self.len() * (size_of::<V>() + size_of::<K>() + size_of::<usize>())
+        }
+    }
+}
+
+impl<K, V> MallocSizeOf for std::collections::BTreeMap<K, V>
+where
+    K: Eq + Hash + MallocSizeOf,
+    V: MallocSizeOf,
+{
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        let mut n = self.shallow_size_of(ops);
+        for (k, v) in self.iter() {
+            n += k.size_of(ops);
+            n += v.size_of(ops);
+        }
+        n
+    }
+}
+
 impl<K, V, S> MallocShallowSizeOf for hashglobe::hash_map::HashMap<K, V, S>
 where
     K: Eq + Hash,

components/script/dom/formdata.rs

@@ -18,19 +18,19 @@ use dom::globalscope::GlobalScope;
 use dom::htmlformelement::{HTMLFormElement, FormDatumValue, FormDatum};
 use dom_struct::dom_struct;
 use html5ever::LocalName;
-use std::collections::HashMap;
-use std::collections::hash_map::Entry::{Occupied, Vacant};
+use std::collections::BTreeMap;
+use std::collections::btree_map::Entry::{Occupied, Vacant};
 use std::iter;
 
 #[dom_struct]
 pub struct FormData {
     reflector_: Reflector,
-    data: DomRefCell<HashMap<LocalName, Vec<FormDatum>>>,
+    data: DomRefCell<BTreeMap<LocalName, Vec<FormDatum>>>,
 }
 
 impl FormData {
     fn new_inherited(opt_form: Option<&HTMLFormElement>) -> FormData {
-        let mut hashmap: HashMap<LocalName, Vec<FormDatum>> = HashMap::new();
+        let mut hashmap: BTreeMap<LocalName, Vec<FormDatum>> = BTreeMap::new();
 
         if let Some(form) = opt_form {
             for datum in form.get_form_dataset(None) {