Allow setting id, class, style without a full reflow

Instead we do selector matching again, then diff the style structs to set the
"restyle damage" bits which are used to prune reflow traversals.

Also don't force a reflow when timers finish, because individual DOM methods
should already take care of that.

src/components/script/script_task.rs

@@ -588,9 +588,6 @@ impl ScriptTask {
                                  &rval);
 
         }
-        // We don't know what the script changed, so for now we will do a total redisplay.
-        page.damage(ContentChangedDocumentDamage);
-        page.reflow(ReflowForDisplay, self.chan.clone(), self.compositor);
     }
 
     /// Handles a notification that reflow completed.