Add a microbenchmark for simple block layout

sky/benchmarks/resources/runner.sky

 <script>
 function PerfRunner(options) {
   this.unit_ = options.unit || "ms";
   this.iterationsRemaining_ = options.iterations || 10;
   this.results_ = [];
+  this.setup_ = options.setup;
 }
 
 PerfRunner.prototype.log = function(line) {
   console.log(line);
 };
 
 PerfRunner.prototype.recordResult = function(result) {
   console.log(result);
   this.results_.push(result);
 };
 
 PerfRunner.prototype.runAsync = function(test) {
   var self = this;
   window.setTimeout(function() {
+    if (self.setup_) {
+      var setup = self.setup_;
+      setup();
+    }
+
     var startTime = Date.now();
     test(function() {
       var endTime = Date.now();
+
       self.recordResult(endTime - startTime);
       if (--self.iterationsRemaining_ > 0)
         self.runAsync(test);
       else
         self.finish();
     });
   });
 };
 
 PerfRunner.prototype.computeStatistics = function() {
@@ skipping 39 matching lines @@
   this.log("min " + stats.min + " " + stats.unit);
   this.log("max " + stats.max + " " + stats.unit);
 };
 
 PerfRunner.prototype.finish = function () {
   this.logStatistics("Time:");
 }
 
 module.exports = PerfRunner;
 </script>