Add comments explaining Task Manager idle wakeups computation.

base/process/process_metrics.cc

Index: base/process/process_metrics.cc
diff --git a/base/process/process_metrics.cc b/base/process/process_metrics.cc
index cc6532c8dca15c233044c24c4f5e0e3557ef99db..d955e0769e793e9e2ed6d7f8f49278a521e91a48 100644
--- a/base/process/process_metrics.cc
+++ b/base/process/process_metrics.cc
@@ -86,7 +86,7 @@ int ProcessMetrics::CalculateIdleWakeupsPerSecond(
 
   // Round to average wakeups per second.
   int64_t wakeups_delta_for_ms = wakeups_delta * Time::kMicrosecondsPerSecond;
-  return (wakeups_delta_for_ms + time_delta / 2) / time_delta;

[brucedawson, 2017/02/02 00:55:29]

This term is presumably (see comment on line 87) to round the value to nearest.
Otherwise the result is truncated, leading to systemic bias.

Adding the divisor divided by two is equivalent to going:

    (int)(((double)wakeups_delta_for_ms / time_delta) + 0.5)

but adding time_delta / 2 avoids converting to floating-point and back.

It's easiest to think of it as being equivalent (modulo computer arithmetic
quirks) to:

wakeups_delta_for_ms / time_delta + (time_delta / 2) / time_delta

[shrike, 2017/02/02 16:12:50]

Thank you brucedawson@. I saw the comment about rounding but didn't understand
it. This is a clever approach to adding 1/2, but I don't think it's clear, and I
dislike that unit-wise it doesn't make sense. I'm going to expand the comment.

+  return wakeups_delta_for_ms / time_delta;
 }
 #else
 int ProcessMetrics::GetIdleWakeupsPerSecond() {