Add comments explaining Task Manager idle wakeups computation.

base/process/process_metrics.cc

 // 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.
 
 #include "base/process/process_metrics.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/values.h"
@@ skipping 66 matching lines @@
   int64_t wakeups_delta = absolute_idle_wakeups - last_absolute_idle_wakeups_;
   int64_t time_delta = (time - last_idle_wakeups_time_).InMicroseconds();
   if (time_delta == 0) {
     NOTREACHED();
     return 0;
   }
 
   last_idle_wakeups_time_ = time;
   last_absolute_idle_wakeups_ = absolute_idle_wakeups;
 
-  // Round to average wakeups per second.
   int64_t wakeups_delta_for_ms = wakeups_delta * Time::kMicrosecondsPerSecond;
+  // Round the result up by adding 1/2 (the second term resolves to 1/2 without
+  // dropping down into floating point).

[brucedawson, 2017/02/02 19:52:03]

It's funny how the set of idioms that are obvious and not needing explanation
varies between programmers. I agree that this one is non-obvious to enough
readers to justify a more specific comment.

   return (wakeups_delta_for_ms + time_delta / 2) / time_delta;
 }
 #else
 int ProcessMetrics::GetIdleWakeupsPerSecond() {
   NOTIMPLEMENTED();  // http://crbug.com/120488
   return 0;
 }
 #endif  // defined(OS_MACOSX) || defined(OS_LINUX)
 
 }  // namespace base