Plumb display metrics into SkyView

This CL teaches SkyView the width, height, and device pixel ratio of the
display. In the future, we'll want some sort of notification system for when
these values change.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1139823010

examples/raw/hello_world.dart

@@ -8,8 +8,8 @@ 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;