rule tree: Remove unsound transmute()

RuleTreeDeclarationsIterator would yield &PropertyDeclaration
with the lifetime of the rule tree, but the reference would only
be valid as long as the iterator was holding the corresponding lock.

The lock would be released when the iterator’s `.next()` method
moves to the next rule tree node, or when  the iterator is dropped.

Also change apply_declaration to not require a `Clone` iterator
(since closures unfortunately don’t implement `Clone`).
Instead have it take a callable that returns a fresh iterator.

ports/geckolib/glue.rs

@@ -131,15 +131,14 @@ pub extern "C" fn Servo_RestyleWithAddedDeclaration(declarations: RawServoDeclar
                                                     previous_style: ServoComputedValuesBorrowed)
   -> ServoComputedValuesStrong
 {
-    use style::declarations_iterators::RawDeclarationsIterator;
-
     let previous_style = ComputedValues::as_arc(&previous_style);
     let declarations = RwLock::<PropertyDeclarationBlock>::as_arc(&declarations);
 
     let guard = declarations.read();
 
-    let declarations =
-        RawDeclarationsIterator::new(&guard.declarations);
+    let declarations = || {
+        guard.declarations.iter().rev().map(|&(ref decl, _importance)| decl)
+    };
 
     // FIXME (bug 1303229): Use the actual viewport size here
     let computed = apply_declarations(Size2D::new(Au(0), Au(0)),