Add Canvas.drawPicture

I wrote another copy of paint_element_into_displaylist
using this new technology.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1129353010

examples/raw/draw_picture_into_canvas.sky

+<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 stampRecorder = new PictureRecorder(width, height);
+  root.paint(stampRecorder);
+  Picture stamp = stampRecorder.endRecording();
+
+  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);
+  recorder.drawPicture(stamp);
+  recorder.translate(10.0, 10.0);
+  recorder.drawPicture(stamp);
+  recorder.translate(10.0, 10.0);
+  recorder.drawPicture(stamp);
+  document.rootPicture = recorder.endRecording();
+}
+</script>