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/servoparser/xml.rs

@@ -17,6 +17,7 @@ use dom::htmlscriptelement::HTMLScriptElement;
 use dom::node::Node;
 use dom::processinginstruction::ProcessingInstruction;
 use dom::text::Text;
+use dom::virtualmethods::vtable_for;
 use html5ever::tokenizer::buffer_queue::BufferQueue;
 use html5ever_atoms::{Prefix, QualName};
 use js::jsapi::JSTracer;
@@ -196,4 +197,9 @@ impl TreeSink for Sink {
             NextParserState::Continue
         }
     }
+
+    fn pop(&mut self, node: Self::Handle) {
+        let node = Root::from_ref(&*node);
+        vtable_for(&node).pop();
+    }
 }