Properly insert text in document when parsing

If the last child of a node is a Text child and we are inserting text in that node,
we need to append it to that Text child. Doing that means that
HTMLStyleElement::children_changed gets called less frequently, but that's not a
problem during parsing thanks to the pop hook.

components/script/dom/virtualmethods.rs

@@ -124,6 +124,14 @@ pub trait VirtualMethods {
             s.cloning_steps(copy, maybe_doc, clone_children);
         }
     }
+
+    /// Called on an element when it is popped off the stack of open elements
+    /// of a parser.
+    fn pop(&self) {
+        if let Some(ref s) = self.super_type() {
+            s.pop();
+        }
+    }
 }
 
 /// Obtain a VirtualMethods instance for a given Node-derived object. Any