[telemetry] Fix types and type checks of numeric results.

tools/perf/metrics/media.js

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains common utilities to find video/audio elements on a page
 // and collect metrics for each.
 
 (function() {
   // MediaMetric class responsible for collecting metrics on a media element.
   // It attaches required event listeners in order to collect different metrics.
@@ skipping 137 matching lines @@
     this.start();
   }
 
   Timer.prototype = {
     start: function() {
       this.start_ = getCurrentTime();
     },
 
     stop: function() {
       // Return delta time since start in millisecs.
-      return ((getCurrentTime() - this.start_)).toFixed(3);
+      return Math.round((getCurrentTime() - this.start_) * 1000) / 1000;
     }
   };
 
   function checkElementIsNotBound(element) {
     if (!element)
       return;
     if (getMediaMetric(element))
       throw new Error('Can not create MediaMetric for same element twice.');
   }
 
@@ skipping 31 matching lines @@
       metrics.push(window.__mediaMetrics[i].getSummary());
     return metrics;
   }
 
   window.__globalCounter = 0;
   window.__mediaMetrics = [];
   window.__getMediaMetric = getMediaMetric;
   window.__getAllMetrics = getAllMetrics;
   window.__createMediaMetricsForDocument = createMediaMetricsForDocument;
 })();