Plumb display metrics into SkyView

sky/examples/raw/hello_world.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import "dart:math";
 import 'dart:sky';
 
 void main() {
   print("Hello, world");
 
-  double width = 500.0;
-  double height = 500.0;
+  double width = view.width;
+  double height = view.height;
 
   PictureRecorder recorder = new PictureRecorder(width, height);
   double radius = min(width, height) * 0.45;
 
   Paint paint = new Paint()..setARGB(255, 0, 255, 0);
 
   recorder.drawCircle(width / 2, height / 2, radius, paint);
 
   print("Storing picture");
   view.picture = recorder.endRecording();
 
   print("Scheduling paint");
   view.schedulePaint();
 }