Report the duration of sampled tasks with histogram.

components/scheduler/renderer/task_duration_reporter_unittest.cc

+// Copyright 2016 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.
+
+#include "components/scheduler/renderer/task_duration_reporter.h"
+
+#include "base/test/histogram_tester.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace scheduler {
+TEST(TaskDurationReporterTest, LoggingDuration) {
+  base::HistogramTester tester;
+
+  TaskDurationReporter tdr;

[tdresser, 2016/07/27 18:04:25]

task_duration_reporter

We try to avoid short acronyms in general.

[sunyunjia, 2016/07/27 19:51:04]

Done.

+  for (int i = 1; i <= 101; ++i) {
+    int millisecond = 5 * 1000;

[tdresser, 2016/07/27 18:04:25]

Settings this to 5 * 1000 seems a bit odd.

I'd probably just go with 1 millisecond most of the type, and then 2
milliseconds on the 100th run, or something like that.

[majidvp, 2016/07/27 18:34:03]

nit: it is better to use a descriptive name such as 
"duration_ms" instead of just millisecond. In fact you 
can initialize duration here and not have two variables.

[sunyunjia, 2016/07/27 19:51:04]

Done.

[sunyunjia, 2016/07/27 19:51:04]

Done.

+    if (i == 100) {
+      millisecond = 1;
+    }
+    base::TimeDelta duration = base::TimeDelta::FromMilliseconds(millisecond);
+    tdr.ReportSampleDuration(duration);
+  }
+  tester.ExpectUniqueSample("RendererScheduler.TaskDuration", 1, 1);
+}
+}