Add a sketch of a test_perf script

sky/benchmarks/layout/simple-blocks.html

-<sky>
-<import src="../resources/runner.sky" as="PerfRunner" />
-<style>
-div {
-  height: 10px;
-}
-span {
-  display: inline;
-}
-</style>
-<div id='content'></div>
-<script>
-var content = document.getElementById('content');
-
-var out = [];
-for (var i = 0; i < 1000; i++) {
-  var div = document.createElement('div');
-  div.appendChild(document.createElement('span')).appendChild(document.createTextNode('foo'));
-  div.appendChild(document.createTextNode(' '));
-  div.appendChild(document.createElement('span')).appendChild(document.createTextNode('bar'));
-  content.appendChild(div);
-}
-
-var b = document.querySelector('sky');
-
-var runner = new PerfRunner({
-  setup: function() {
-    b.style.width = '210px';
-    getComputedStyle(b).color;
-    b.offsetHeight;
-
-    b.style.width = '200px';
-    getComputedStyle(b).color;
-  },
-  iterations: 10,
-  unit: 'ms',
-});
-
-runner.runAsync(function(done) {
-  b.offsetHeight;
-  done();
-});
-</script>
-</sky>