Fix clipping of content outside the bounds of the window.

sky/tests/clipping/clipping-transforms.sky

-<foo>
 <import src="../resources/run-after-display.sky" />
 
-<img src="http://storage.googleapis.com/mojo/sky-pngs/9a7b59f74e2bbcd0d0ceee17ead05b0da64a2af7" />
+<style>
+transformer {
+  transform: translateY(-500px);
+}
+child {
+  opacity: 0.5;
+  background-image: linear-gradient(green, yellow);
+  height: 1900px;
+}
+</style>
+<transformer>
+  <child />
+</transformer>
 
 <script>
 import "dart:sky";
 import "dart:sky.internals" as internals;
 
 void main() {
   window.addEventListener('load', (_) {
     runAfterDisplay(() {
       internals.notifyTestComplete("");
     });
   });
 }
 </script>
-</foo>