Store pristine element state rather than a set of changes.

This is the strategy we'll need to take for attributes, and so this change
puts us in a position to handle attributes and state the same way.

This does mean that we stop taking care to track the situations where our
state has reverted to the original state, with no net change. I think that's
probably of negligible value though.

components/style/selector_matching.rs

@@ -174,10 +174,10 @@ impl Stylist {
 
     pub fn restyle_hint_for_state_change<E>(&self, element: &E,
                                             current_state: ElementState,
-                                            state_change: ElementState)
+                                            old_state: ElementState)
                                             -> RestyleHint
                                             where E: Element + Clone {
-        self.state_deps.compute_hint(element, current_state, state_change)
+        self.state_deps.compute_hint(element, current_state, old_state)
     }
 
     pub fn set_device(&mut self, device: Device) {