Port rendering_stats' implementation to javascript

tracing/tracing/base/math.html

Index: tracing/tracing/base/math.html
diff --git a/tracing/tracing/base/math.html b/tracing/tracing/base/math.html
index b6f7fd58d061b7d66cec3b548ae5e8cf8c769dd3..1ce443704f84769cad45db14e54c4bb5a291611e 100644
--- a/tracing/tracing/base/math.html
+++ b/tracing/tracing/base/math.html
@@ -11,6 +11,12 @@ found in the LICENSE file.
 'use strict';
 
 tr.exportTo('tr.b', function() {
+  function round(number, opt_ndigits) {
+    opt_ndigits = opt_ndigits || 1;

[benjhayden, 2015/09/25 02:01:18]

This prevents setting opt_ndigits = 0. I don't know if that's an expected use
case, or if you'd expect callers to just use Math.round().

+    var k = Math.pow(10, opt_ndigits);
+    return Math.round(number * k) / k;
+  }
+
   function clamp(x, lo, hi) {
     return Math.min(Math.max(x, lo), hi);
   }
@@ -104,6 +110,7 @@ tr.exportTo('tr.b', function() {
   }
 
   return {
+    round: round,
     clamp: clamp,
     lerp: lerp,
     normalize: normalize,