Make it possible to Paint elements into a display list.

Currently said elements need to be in the DOM and have
already been laid out for this to work, but follow-up patches
will remove these restrictions.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1144673002

examples/raw/paint_element_into_displaylist.sky

@@ -0,0 +1,24 @@
+<script>
+import "dart:sky";
+
+void main() {
+  var root = document.createElement('div');
+  root.style['display'] = 'paragraph';
+  root.appendChild(new Text('Hello World'));
+  document.appendChild(root);
+  root.offsetWidth; // force layout.
+
+  double width = window.innerWidth.toDouble();
+  double height = window.innerHeight.toDouble();
+  PictureRecorder recorder = new PictureRecorder(width, height);
+  Paint paint = new Paint()..setARGB(255, 0, 255, 0);
+  recorder.drawCircle(50.0, 50.0, 50.0, paint);
+  recorder.translate(10.0, 10.0);
+  root.paint(recorder);
+  recorder.translate(10.0, 10.0);
+  root.paint(recorder);
+  recorder.translate(10.0, 10.0);
+  root.paint(recorder);
+  document.rootPicture = recorder.endRecording();
+}
+</script>