Report the duration of sampled tasks with histogram.

components/scheduler/renderer/renderer_task_duration.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 "base/metrics/histogram_macros.h"
+#include "components/scheduler/renderer/renderer_task_duration.h"

[majidvp, 2016/07/27 13:50:59]

The class's own header should be included first. Then followed by  a blank 
line and other includes.

See:
https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

[sunyunjia, 2016/07/27 17:28:42]

Done.

+
+namespace scheduler {
+  RendererTaskDuration::RendererTaskDuration()
+      : sample_count_(0) {

[majidvp, 2016/07/27 13:50:59]

The initializer list should be on the same line as it falls below the 80
characters limit.

[sunyunjia, 2016/07/27 17:28:42]

Done.

+  }
+
+  RendererTaskDuration::~RendererTaskDuration() {

[majidvp, 2016/07/27 13:50:59]

Move closing brace to the same line for this empty function i.e., {}
Same for above.

https://google.github.io/styleguide/cppguide.html#Horizontal_Whitespace

[sunyunjia, 2016/07/27 17:28:42]

Done.

+  }
+
+  void RendererTaskDuration::ReportSampleDuration(base::TimeDelta duration) {
+    ++sample_count_;
+    if (sample_count_ % kSampleInterval == 0) {

[majidvp, 2016/07/27 13:50:59]

Given that we reset the sample count to 0. 
Should this be just sample_count_  == kSampleInterval ?

[sunyunjia, 2016/07/27 17:28:42]

Done.

+      UMA_HISTOGRAM_TIMES("RendererScheduler.TaskDuration",

[majidvp, 2016/07/27 13:50:59]

UMA_HISTGRAM_TIMES range is 1ms - 10seconds. This means that 
all long tasks larger than 10ms will get into the same bucket.
I think this is good enough but if we want a better resolution for long
tasks we may want to consider a larger range. I think it is good to 
2x check with tdresser@.

+                          duration);

[majidvp, 2016/07/27 13:50:59]

The line break is not needed. Falls within the 80 characters limits.

[sunyunjia, 2016/07/27 17:28:42]

Done.

+      sample_count_ = 0;
+    }
+  }
+}